MSC2705: Animated thumbnails for media (#2705)
* Proposal to support animated thumbnails
* add implementation note
* Apply suggestions from code review
Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>
* Clean up the MSC
* Appease the keepers of mimetypes
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Co-authored-by: Alexey Rusakov <Kitsune-Ral@users.sf.net>
