RestrictOutboundClipboard kills all of test/file/image
transfer via clipboard.
For controlling each content type behavior,
clipboard_xevent is not appropriate place to block respectively.
Instead, in clipboard_event_selection_notify, these media type
will be blocked which depends on the following configurations in
sesman.ini [Security] section.
* RestrictOutboundClipboard=text
* RestrictOutboundClipboard=file
* RestrictOutboundClipboard=image
You can also set comma separated list
* RestrictOutboundClipboard=text, file, image
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