netsurf/content/mimesniff.c

/*
 * Copyright 2011 John-Mark Bell <jmb@netsurf-browser.org>
 *
 * This file is part of NetSurf, http://www.netsurf-browser.org/
 *
 * NetSurf is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; version 2 of the License.
 *
 * NetSurf is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

/** \file
 * MIME type sniffer (implementation)
 *
 * Spec version: 2011-11-27
 */

#include <string.h>
#include <strings.h>

#include "content/content_factory.h"
#include "content/llcache.h"
#include "content/mimesniff.h"
#include "utils/http.h"
#include "utils/utils.h"

struct map_s {
	const uint8_t *sig;
	size_t len;
	bool safe;
	lwc_string **type;
};

static lwc_string *unknown_unknown;
static lwc_string *application_unknown;
static lwc_string *any;
static lwc_string *text_xml;
static lwc_string *application_xml;
static lwc_string *text_html;
static lwc_string *text_plain;
static lwc_string *application_octet_stream;
static lwc_string *image_gif;
static lwc_string *image_png;
static lwc_string *image_jpeg;
static lwc_string *image_bmp;
static lwc_string *image_vnd_microsoft_icon;
static lwc_string *image_webp;
static lwc_string *application_rss_xml;
static lwc_string *application_atom_xml;
static lwc_string *audio_wave;
static lwc_string *application_ogg;
static lwc_string *video_webm;
static lwc_string *application_x_rar_compressed;
static lwc_string *application_zip;
static lwc_string *application_x_gzip;
static lwc_string *application_postscript;
static lwc_string *application_pdf;
static lwc_string *video_mp4;
static lwc_string *image_svg;

nserror mimesniff_init(void)
{
	lwc_error lerror;

#define SINIT(v, s) \
	lerror = lwc_intern_string(s, SLEN(s), &v); \
	if (lerror != lwc_error_ok) \
		return NSERROR_NOMEM

	SINIT(unknown_unknown,              "unknown/unknown");
	SINIT(application_unknown,          "application/unknown");
	SINIT(any,                          "*/*");
	SINIT(text_xml,                     "text/xml");
	SINIT(application_xml,              "application/xml");
	SINIT(text_html,                    "text/html");
	SINIT(text_plain,                   "text/plain");
	SINIT(application_octet_stream,     "application/octet-stream");
	SINIT(image_gif,                    "image/gif");
	SINIT(image_png,                    "image/png");
	SINIT(image_jpeg,                   "image/jpeg");
	SINIT(image_bmp,                    "image/bmp");
	SINIT(image_vnd_microsoft_icon,     "image/vnd.microsoft.icon");
	SINIT(image_webp,                   "image/webp");
	SINIT(application_rss_xml,          "application/rss+xml");
	SINIT(application_atom_xml,         "application/atom+xml");
	SINIT(audio_wave,                   "audio/wave");
	SINIT(application_ogg,              "application/ogg");
	SINIT(video_webm,                   "video/webm");
	SINIT(application_x_rar_compressed, "application/x-rar-compressed");
	SINIT(application_zip,              "application/zip");
	SINIT(application_x_gzip,           "application/x-gzip");
	SINIT(application_postscript,       "application/postscript");
	SINIT(application_pdf,              "application/pdf");
	SINIT(video_mp4,                    "video/mp4");
	SINIT(image_svg,                    "image/svg+xml");
#undef SINIT

	return NSERROR_OK;
}

void mimesniff_fini(void)
{
	lwc_string_unref(image_svg);
	lwc_string_unref(video_mp4);
	lwc_string_unref(application_pdf);
	lwc_string_unref(application_postscript);
	lwc_string_unref(application_x_gzip);
	lwc_string_unref(application_zip);
	lwc_string_unref(application_x_rar_compressed);
	lwc_string_unref(video_webm);
	lwc_string_unref(application_ogg);
	lwc_string_unref(audio_wave);
	lwc_string_unref(application_atom_xml);
	lwc_string_unref(application_rss_xml);
	lwc_string_unref(image_webp);
	lwc_string_unref(image_vnd_microsoft_icon);
	lwc_string_unref(image_bmp);
	lwc_string_unref(image_jpeg);
	lwc_string_unref(image_png);
	lwc_string_unref(image_gif);
	lwc_string_unref(application_octet_stream);
	lwc_string_unref(text_plain);
	lwc_string_unref(text_html);
	lwc_string_unref(application_xml);
	lwc_string_unref(text_xml);
	lwc_string_unref(any);
	lwc_string_unref(application_unknown);
	lwc_string_unref(unknown_unknown);
}

static bool mimesniff__has_binary_octets(const uint8_t *data, size_t len)
{
	const uint8_t *end = data + len;

	while (data != end) {
		const uint8_t c = *data;

		/* Binary iff in C0 and not ESC, CR, FF, LF, HT */
		if (c <= 0x1f && c != 0x1b && c != '\r' && c != '\f' && 
				c != '\n' && c != '\t')
			break;

		data++;
	}

	return data != end;
}

static nserror mimesniff__match_mp4(const uint8_t *data, size_t len,
		lwc_string **effective_type)
{
	uint32_t box_size, i;

	/* ISO/IEC 14496-12:2008 $4.3 says (effectively):
	 *
	 * struct ftyp_box {
	 *   uint32_t size; (in octets, including size+type words)
	 *   uint32_t type; (== 'ftyp')
	 *   uint32_t major_brand;
	 *   uint32_t minor_version;
	 *   uint32_t compatible_brands[];
	 * }
	 *
	 * Note 1: A size of 0 implies that the length of the box is designated 
	 * by the remaining input data (and thus may only occur in the last 
	 * box in the input). We'll reject this below, as it's pointless 
	 * sniffing input that contains no boxes other than 'ftyp'.
	 *
	 * Note 2: A size of 1 implies an additional uint64_t field after 
	 * the type which contains the extended box size. We'll reject this, 
	 * too, as it implies a minimum of (2^32 - 24) / 4 compatible brands, 
	 * which is decidely unlikely.
	 */

	/* 12 reflects the minimum number of octets needed to sniff useful 
	 * information out of an 'ftyp' box (i.e. the size, type, 
	 * and major_brand words). */
	if (len < 12)
		return NSERROR_NOT_FOUND;

	/* Box size is big-endian */
	box_size = (data[0] << 24) | (data[1] << 16) | (data[2] << 8) | data[3];

	/* Require that we can read the entire box, and reject bad box sizes */
	if (len < box_size || box_size % 4 != 0)
		return NSERROR_NOT_FOUND;

	/* Ensure this is an 'ftyp' box */
	if (data[4] != 'f' || data[5] != 't' || 
			data[6] != 'y' || data[7] != 'p')
		return NSERROR_NOT_FOUND;

	/* Check if major brand begins with 'mp4' */
	if (data[8] == 'm' && data[9] == 'p' && data[10] == '4') {
		*effective_type = lwc_string_ref(video_mp4);
		return NSERROR_OK;
	}

	/* Search each compatible brand in the box for "mp4" */
	for (i = 16; i <= box_size - 4; i += 4) {
		if (data[i] == 'm' && 
		    data[i+1] == 'p' && 
		    data[i+2] == '4') {
			*effective_type = lwc_string_ref(video_mp4);
			return NSERROR_OK;
		}
	}

	return NSERROR_NOT_FOUND;
}

static nserror mimesniff__match_unknown_ws(const uint8_t *data, size_t len,
		lwc_string **effective_type)
{
#define SIG(t, s, x) { (const uint8_t *) s, SLEN(s), x, t }
	static const struct map_s ws_exact_match_types[] = {
		SIG(&text_xml, "<?xml", false),
		{ NULL, 0, false, NULL }
	};

	static const struct map_s ws_inexact_match_types[] = {
		SIG(&text_html, "<!DOCTYPE HTML", false),
		SIG(&text_html, "<HTML",          false),
		SIG(&text_html, "<HEAD",          false),
		SIG(&text_html, "<SCRIPT",        false),
		SIG(&text_html, "<IFRAME",        false),
		SIG(&text_html, "<H1",            false),
		SIG(&text_html, "<DIV",           false),
		SIG(&text_html, "<FONT",          false),
		SIG(&text_html, "<TABLE",         false),
		SIG(&text_html, "<A",             false),
		SIG(&text_html, "<STYLE",         false),
		SIG(&text_html, "<TITLE",         false),
		SIG(&text_html, "<B",             false),
		SIG(&text_html, "<BODY",          false),
		SIG(&text_html, "<BR",            false),
		SIG(&text_html, "<P",             false),
		SIG(&text_html, "<!--",           false),
		{ NULL, 0, false, NULL }
	};
#undef SIG
	const uint8_t *end = data + len;
	const struct map_s *it;

	/* Skip leading whitespace */
	while (data != end) {
		const uint8_t c = *data;

		if (c != '\t' && c != '\n' && c != '\f' && 
				c != '\r' && c != ' ')
			break;

		data++;
	}

	if (data == end)
		return NSERROR_NOT_FOUND;

	len = end - data;

	for (it = ws_exact_match_types; it->sig != NULL; it++) {
		if (it->len <= len && memcmp(data, it->sig, it->len) == 0) {
			*effective_type = lwc_string_ref(*it->type);
			return NSERROR_OK;
		}
	}

	for (it = ws_inexact_match_types; it->sig != NULL; it++) {
		/* +1 for trailing space or > */
		if (len < it->len + 1)
			continue;

		if (strncasecmp((const char *) data, 
				(const char *) it->sig, it->len) == 0 && 
				(data[it->len] == ' ' || 
				data[it->len] == '>')) {
			*effective_type = lwc_string_ref(*it->type);
			return NSERROR_OK;
		}
	}

	return NSERROR_NOT_FOUND;
}

static nserror mimesniff__match_unknown_bom(const uint8_t *data, size_t len,
		lwc_string **effective_type)
{
#define SIG(t, s, x) { (const uint8_t *) s, SLEN(s), x, t }
	static const struct map_s bom_match_types[] = {
		SIG(&text_plain, "\xfe\xff",     false),
		SIG(&text_plain, "\xff\xfe",     false),
		SIG(&text_plain, "\xef\xbb\xbf", false),
		{ NULL, 0, false, NULL }
	};
#undef SIG
	const struct map_s *it;

	for (it = bom_match_types; it->sig != NULL; it++) {
		if (it->len <= len && memcmp(data, it->sig, it->len) == 0) {
			*effective_type = lwc_string_ref(*it->type);
			return NSERROR_OK;
		}
	}

	return NSERROR_NOT_FOUND;
}

static nserror mimesniff__match_unknown_riff(const uint8_t *data, size_t len,
		lwc_string **effective_type)
{
#define SIG(t, s, x) { (const uint8_t *) s, SLEN(s), x, t }
	static const struct map_s riff_match_types[] = {
		SIG(&image_webp, "WEBPVP", true),
		SIG(&audio_wave, "WAVE",   true),
		{ NULL, 0, false, NULL }
	};
#undef SIG
	const struct map_s *it;

	for (it = riff_match_types; it->sig != NULL; it++) {
		if (it->len + SLEN("RIFF????") <= len && 
				memcmp(data, "RIFF", SLEN("RIFF")) == 0 &&
				memcmp(data + SLEN("RIFF????"), 
						it->sig, it->len) == 0) {
			*effective_type = lwc_string_ref(*it->type);
			return NSERROR_OK;
		}
	}

	return NSERROR_NOT_FOUND;
}

static nserror mimesniff__match_unknown_exact(const uint8_t *data, size_t len,
		bool allow_unsafe, lwc_string **effective_type)
{
#define SIG(t, s, x) { (const uint8_t *) s, SLEN(s), x, t }
	static const struct map_s exact_match_types[] = {
		SIG(&image_gif,                    "GIF87a",            true),
		SIG(&image_gif,                    "GIF89a",            true),
		SIG(&image_png,                    "\x89PNG\r\n\x1a\n", true),
		SIG(&image_jpeg,                   "\xff\xd8\xff",      true),
		SIG(&image_bmp,                    "BM",                true),
		SIG(&image_vnd_microsoft_icon,     "\x00\x00\x01\x00",  true),
		SIG(&application_ogg,              "OggS\x00",          true),
		SIG(&video_webm,                   "\x1a\x45\xdf\xa3",  true),
		SIG(&application_x_rar_compressed, "Rar \x1a\x07\x00",  true),
		SIG(&application_zip,              "PK\x03\x04",        true),
		SIG(&application_x_gzip,           "\x1f\x8b\x08",      true),
		SIG(&application_postscript,       "%!PS-Adobe-",       true),
		SIG(&application_pdf,              "%PDF-",             false),
		{ NULL, 0, false, NULL }
	};
#undef SIG
	const struct map_s *it;

	for (it = exact_match_types; it->sig != NULL; it++) {
		if (it->len <= len && memcmp(data, it->sig, it->len) == 0 &&
				(allow_unsafe || it->safe)) {
			*effective_type = lwc_string_ref(*it->type);
			return NSERROR_OK;
		}
	}

	return NSERROR_NOT_FOUND;
}

static nserror mimesniff__match_unknown(const uint8_t *data, size_t len,
		bool allow_unsafe, lwc_string **effective_type)
{
	if (mimesniff__match_unknown_exact(data, len, allow_unsafe, 
			effective_type) == NSERROR_OK)
		return NSERROR_OK;

	if (mimesniff__match_unknown_riff(data, len, 
			effective_type) == NSERROR_OK)
		return NSERROR_OK;

	if (allow_unsafe == false)
		return NSERROR_NOT_FOUND;

	if (mimesniff__match_unknown_bom(data, len,
			effective_type) == NSERROR_OK)
		return NSERROR_OK;

	if (mimesniff__match_unknown_ws(data, len,
			effective_type) == NSERROR_OK)
		return NSERROR_OK;

	if (mimesniff__match_mp4(data, len, effective_type) == NSERROR_OK)
		return NSERROR_OK;

	return NSERROR_NOT_FOUND;
}

static nserror mimesniff__compute_unknown(const uint8_t *data, size_t len,
		lwc_string **effective_type)
{
	if (data == NULL)
		return NSERROR_NEED_DATA;

	len = min(len, 512);

	if (mimesniff__match_unknown(data, len, true, 
			effective_type) == NSERROR_OK)
		return NSERROR_OK;

	if (mimesniff__has_binary_octets(data, len) == false) {
		/* No binary octets => text/plain */
		*effective_type = lwc_string_ref(text_plain);
		return NSERROR_OK;
	}

	*effective_type = lwc_string_ref(application_octet_stream);

	return NSERROR_OK;
}

static nserror mimesniff__compute_text_or_binary(const uint8_t *data, 
		size_t len, lwc_string **effective_type)
{
	if (data == NULL)
		return NSERROR_NEED_DATA;

	len = min(len, 512);

	if (len >= 3 && ((data[0] == 0xfe && data[1] == 0xff) ||
			(data[0] == 0xff && data[1] == 0xfe) ||
			(data[0] == 0xef && data[1] == 0xbb && 
				data[2] == 0xbf))) {
		/* Found a BOM => text/plain */
		*effective_type = lwc_string_ref(text_plain);
		return NSERROR_OK;
	}

	if (mimesniff__has_binary_octets(data, len) == false) {
		/* No binary octets => text/plain */
		*effective_type = lwc_string_ref(text_plain);
		return NSERROR_OK;
	}

	if (mimesniff__match_unknown(data, len, false, 
			effective_type) == NSERROR_OK)
		return NSERROR_OK;

	*effective_type = lwc_string_ref(application_octet_stream);

	return NSERROR_OK;
}

static nserror mimesniff__compute_image(lwc_string *official_type,
		const uint8_t *data, size_t len, lwc_string **effective_type)
{
#define SIG(t, s) { (const uint8_t *) s, SLEN(s), t }
	static const struct it_s {
		const uint8_t *sig;
		size_t len;
		lwc_string **type;
	} image_types[] = {
		SIG(&image_gif,                "GIF87a"),
		SIG(&image_gif,                "GIF89a"),
		SIG(&image_png,                "\x89PNG\r\n\x1a\n"),
		SIG(&image_jpeg,               "\xff\xd8\xff"),
		SIG(&image_bmp,                "BM"),
		SIG(&image_vnd_microsoft_icon, "\x00\x00\x01\x00"),
		{ NULL, 0, NULL }
	};
#undef SIG

	const struct it_s *it;

	if (data == NULL) {
		lwc_string_unref(official_type);
		return NSERROR_NEED_DATA;
	}

	for (it = image_types; it->sig != NULL; it++) {
		if (it->len <= len && memcmp(data, it->sig, it->len) == 0) {
			lwc_string_unref(official_type);
			*effective_type = lwc_string_ref(*it->type);
			return NSERROR_OK;
		}
	}

	/* WebP has a signature that doesn't fit into the above table */
	if (SLEN("RIFF????WEBPVP") <= len && 
			memcmp(data, "RIFF", SLEN("RIFF")) == 0 && 
			memcmp(data + SLEN("RIFF????"), 
					"WEBPVP", SLEN("WEBPVP")) == 0 ) {
		lwc_string_unref(official_type);
		*effective_type = lwc_string_ref(image_webp);
		return NSERROR_OK;
	}

	*effective_type = official_type;

	return NSERROR_OK;
}

static nserror mimesniff__compute_feed_or_html(const uint8_t *data,
		size_t len, lwc_string **effective_type)
{
#define RDF_NS "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
#define RSS_NS "http://purl.org/rss/1.0"

	enum state_e {
		BEFORE_BOM,
		BEFORE_MARKUP,
		MARKUP_START,
		COMMENT_OR_DOCTYPE,
		IN_COMMENT,
		IN_DOCTYPE,
		IN_PI,
		IN_TAG,
		IN_RDF
	} state = BEFORE_BOM;

	bool rdf = false, rss = false;
	const uint8_t *end;

	if (data == NULL)
		return NSERROR_NEED_DATA;

	end = data + min(len, 512);

	while (data < end) {
		const uint8_t c = *data;

#define MATCH(s) SLEN(s) <= (size_t) (end - data) && \
			memcmp(data, s, SLEN(s)) == 0

		switch (state) {
		case BEFORE_BOM:
			if (3 <= end - data && c == 0xef && data[1] == 0xbb && 
					data[2] == 0xbf) {
				data += 3;
			}

			state = BEFORE_MARKUP;
			break;
		case BEFORE_MARKUP:
			if (c == '\t' || c == '\n' || c	== '\r' || c == ' ')
				data++;
			else if (c != '<')
				data = end;
			else {
				state = MARKUP_START;
				data++;
			}
			break;
		case MARKUP_START:
			if (c == '!') {
				state = COMMENT_OR_DOCTYPE;
				data++;
			} else if (c == '?') {
				state = IN_PI;
				data++;
			} else {
				/* Reconsume input */
				state = IN_TAG;
			}
			break;
		case COMMENT_OR_DOCTYPE:
			if (2 <= end - data && c == '-' && data[1] == '-') {
				state = IN_COMMENT;
				data += 2;
			} else {
				/* Reconsume input */
				state = IN_DOCTYPE;
			}
			break;
		case IN_COMMENT:
			if (3 <= end - data && c == '-' && data[1] == '-' &&
					data[2] == '>') {
				state = BEFORE_MARKUP;
				data += 3;
			} else
				data++;
			break;
		case IN_DOCTYPE:
			if (c == '>')
				state = BEFORE_MARKUP;
			data++;
			break;
		case IN_PI:
			if (2 <= end - data && c == '?' && data[1] == '>') {
				state = BEFORE_MARKUP;
				data += 2;
			} else
				data++;
			break;
		case IN_TAG:
			if (MATCH("rss")) {
				*effective_type = 
					lwc_string_ref(application_rss_xml);
				return NSERROR_OK;
			} else if (MATCH("feed")) {
				*effective_type = 
					lwc_string_ref(application_atom_xml);
				return NSERROR_OK;
			} else if (MATCH("rdf:RDF")) {
				state = IN_RDF;
				data += SLEN("rdf:RDF");
			} else
				data = end;
			break;
		case IN_RDF:
			if (MATCH(RSS_NS)) {
				rss = true;
				data += SLEN(RSS_NS);
			} else if (MATCH(RDF_NS)) {
				rdf = true;
				data += SLEN(RDF_NS);
			} else
				data++;

			if (rdf && rss) {
				*effective_type = 
					lwc_string_ref(application_rss_xml);
				return NSERROR_OK;
			}

			break;
		}
#undef MATCH
	}

	*effective_type = lwc_string_ref(text_html);

	return NSERROR_OK;

#undef RSS_NS
#undef RDF_NS
}

/* See mimesniff.h for documentation */
nserror mimesniff_compute_effective_type(llcache_handle *handle,
		const uint8_t *data, size_t len, bool sniff_allowed,
		bool image_only, lwc_string **effective_type)
{
#define S(s) { s, SLEN(s) }
	static const struct tt_s {
		const char *data;
		size_t len;
	} text_types[] = {
		S("text/plain"),
		S("text/plain; charset=ISO-8859-1"),
		S("text/plain; charset=iso-8859-1"),
		S("text/plain; charset=UTF-8"),
		{ NULL, 0 }
	};
#undef S

	const char *content_type_header;
	size_t content_type_header_len;
	http_content_type *ct;
	const struct tt_s *tt;
	bool match;
	nserror error;

	content_type_header = 
			llcache_handle_get_header(handle, "Content-Type");
	if (content_type_header == NULL) {
		if (sniff_allowed == false)
			return NSERROR_NOT_FOUND;

		/* No official type => unknown */
		return mimesniff__compute_unknown(data, len, effective_type);
	}

	error = http_parse_content_type(content_type_header, &ct);
	if (error != NSERROR_OK) {
		if (sniff_allowed == false)
			return NSERROR_NOT_FOUND;

		/* Unparseable => unknown */
		return mimesniff__compute_unknown(data, len, effective_type);
	}

	if (sniff_allowed == false) {
		*effective_type = lwc_string_ref(ct->media_type);
		http_content_type_destroy(ct);
		return NSERROR_OK;
	}

	if (image_only) {
		lwc_string *official_type;

		if (lwc_string_caseless_isequal(ct->media_type, image_svg, 
				&match) == lwc_error_ok && match) {
			*effective_type = lwc_string_ref(image_svg);
			http_content_type_destroy(ct);
			return NSERROR_OK;
		}

		official_type = lwc_string_ref(ct->media_type);
		http_content_type_destroy(ct);
		return mimesniff__compute_image(official_type,
				data, len, effective_type);
	}

	content_type_header_len = strlen(content_type_header);

	/* Look for text types */
	for (tt = text_types; tt->data != NULL; tt++) {
		if (tt->len == content_type_header_len &&
				memcmp(tt->data, content_type_header, 
					content_type_header_len) == 0) {
			http_content_type_destroy(ct);
			return mimesniff__compute_text_or_binary(data, len,
					effective_type);
		}
	}

	/* unknown/unknown, application/unknown, * / * */
	if ((lwc_string_caseless_isequal(ct->media_type, unknown_unknown, 
				&match) == lwc_error_ok && match) ||
			(lwc_string_caseless_isequal(ct->media_type, 
				application_unknown, &match) == lwc_error_ok && 
				match) ||
			(lwc_string_caseless_isequal(ct->media_type, any, 
				&match) == lwc_error_ok && match)) {
		http_content_type_destroy(ct);
		return mimesniff__compute_unknown(data, len, effective_type);
	}

	/* +xml */
	if (lwc_string_length(ct->media_type) > SLEN("+xml") &&
			strncasecmp(lwc_string_data(ct->media_type) + 
				lwc_string_length(ct->media_type) - 
				SLEN("+xml"), 
				"+xml", SLEN("+xml")) == 0) {
		/* Use official type */
		*effective_type = lwc_string_ref(ct->media_type);
		http_content_type_destroy(ct);
		return NSERROR_OK;
	}

	/* text/xml, application/xml */
	if ((lwc_string_caseless_isequal(ct->media_type, text_xml, 
				&match) == lwc_error_ok && match) ||
			(lwc_string_caseless_isequal(ct->media_type, 
				application_xml, &match) == lwc_error_ok && 
				match)) {
		/* Use official type */
		*effective_type = lwc_string_ref(ct->media_type);
		http_content_type_destroy(ct);
		return NSERROR_OK;
	}
	
	/* Image types */
	if (content_factory_type_from_mime_type(ct->media_type) == 
			CONTENT_IMAGE) {
		lwc_string *official_type = lwc_string_ref(ct->media_type);
		http_content_type_destroy(ct);
		return mimesniff__compute_image(official_type,
				data, len, effective_type);
	}

	/* text/html */
	if ((lwc_string_caseless_isequal(ct->media_type, text_html, 
			&match) == lwc_error_ok && match)) {
		http_content_type_destroy(ct);
		return mimesniff__compute_feed_or_html(data, len,
				effective_type);
	}

	/* Use official type */
	*effective_type = lwc_string_ref(ct->media_type);

	http_content_type_destroy(ct);

	return NSERROR_OK;
}