Allow TextSearch to grep XHTML files

...while the setting "Text files only" is active.
Fixes #12539.

Once users are able to set supported MIME types on packaged apps
(which are read-only), it might be a good idea to have TextSearch
check its supported MIME types and examine the target files accordingly.

src/apps/text_search/FileIterator.cpp

@@ -58,6 +58,9 @@ FileIterator::_ExamineFile(BEntry& entry, char* buffer, bool textFilesOnly)
 			return true;
 		}
 	}
+	// Make an exception for XHTML files
+	if (strcmp("application/xhtml+xml", mimeTypeString) == 0)
+		return true;
 
 	return false;
 }