HEIC/HEIF support

This issue has been migrated from [#13767](https://github.com/matrix-org/synapse/issues/13767).

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**Description:**

Apparently someone managed to send an HEIC image from an apple device to a matrix room I participate in. At least thumbnailing HEIC/HEIF ( https://en.wikipedia.org/wiki/High_Efficiency_Image_File_Format ) would be nice. I suppose synapse isn't supposed to be a generic media converter, but that would have been useful as well.

PNGs should be thumbnailed as JPEG https://github.com/matrix-org/synapse/issues/2045 , but there's complaints about WEBP being thumbnailed about JPEG https://github.com/matrix-org/synapse/issues/11840 . I guess it's hard to please everyone.