WEBPs are thumbnailed as JPEGs, losing transparency/animation

[matrixbot]

This issue has been migrated from #11840.

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Description

Exactly as it sounds, WEBPs that have transparency get thumbnailed as JPEGs, making them look terrible. Tracked it down to #4382, where WEBPs were universally marked to use JPEG for thumbnailing instead of PNGs, or WEBPs, though I assume that last one is more of a Synapse limitation than an oversight. Intended behavior is for transparent images to have transparent thumbnails, including WEBPs.

Steps to reproduce

• Post a transparent WEBP into an unencrypted room
• See a terrible blocky black background

Version information

• Homeserver:

Homeserver was matrix.org

• Version: 1.51.0rc1 (b=matrix-org-hotfixes,7977b7f6a)

• Install method: Not sure what they do.

• Platform: Not sure.