AVIF image format support
AVIF (which is also a still image of av1 video compressor) is one of the most modern image formats (in addition to JPEG XL).
Majority of web browsers already support it and modern news websites are using it on regular basis. Sites such as Facebook / Instagram support it (and btw.. render avif uploads much faster than jpg)
If you want to compress / archive your image collection AVIF and JPEG XL are best formats to do that as they are open source. Problems start when you want to share an image from your collection to a Mattermost site.
I know this is not (yet) as requested feature as HEIC support. Dunno if HEIC support is even possible because it can be expensive/complex to license it but AVIF for sure should be easy.
All libraries are already available and solid, it probably is not large changes to Mattermost to get AVIF support.

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Golan Lampi commented
AVIF (which is also a still image of av1 video compressor) is one of the most modern image formats (in addition to JPEG XL).
Majority of web browsers already support it and modern news websites are using it on regular basis. Sites such as Facebook / Instagram support it (and btw.. render avif uploads much faster than jpg)
If you want to compress / archive your image collection AVIF and JPEG XL are best formats to do that as they are open source. Problems start when you want to share an image from your collection to a Mattermost site.
I know this is not (yet) as requested feature as HEIC support. Dunno if HEIC support is even possible because it can be expensive/complex to license it but AVIF for sure should be easy.
All libraries are already available and solid, it probably is not large changes to Mattermost to get AVIF support.