Add new hook for two-step media type checking

Before uploaded media files were checked by extension. This led to
situations when a plugin can support file with specific extension but
doesn't due to lack of codecs, for example. Since the plugin reported
that it supports uploaded file type, the upload was being declared
successful, but transcoding failed.

The failures were not easy to debug.

The change adds a new hook that could allow two-step checking of the
content. The result of the hook execution returns a tuple with
media type name, manager and a callable sniffer, that can be used to
perform probably expensive checks of the content.

Also the change adds implementation of the hook for video.

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