Server can't generate preview images for animated WebP files

[mattermod]

Originally reported by https://github.com/mattermost/mattermost/issues/26704

Repro steps:

1. Post one of the animated webps found at webp.zip

Observed: The file can be uploaded and posted as an attachment, but the preview image for it will be blank. With debug logging enabled, errors like the following will be logged on upload and when viewing the post:

{"timestamp":"2024-04-29 14:57:26.650 -04:00","level":"error","msg":"Unable to decode image","caller":"app/file.go:899","path":"/api/v4/files","request_id":"jkky79m8kjnnzet813cundpj1r","ip_addr":"127.0.0.1","user_id":"d8njkgw3yifzjee3bejpkqb8kw","method":"POST","file_name":"panic.webp","error":"imaging: failed to decode image: webp: invalid format"}
{"timestamp":"2024-04-29 14:57:26.871 -04:00","level":"debug","msg":"generateMiniPreview: prepareImage failed","caller":"app/file.go:1192","path":"/api/v4/files/twq74tqr8bdzzxzthxzgfn6kqo/thumbnail","request_id":"za8bydua838adryqweinhzhmzo","ip_addr":"127.0.0.1","user_id":"d8njkgw3yifzjee3bejpkqb8kw","method":"GET","error":"prepareImage: failed to decode image: imaging: failed to decode image: webp: invalid format","fileinfo_id":"twq74tqr8bdzzxzthxzgfn6kqo","channel_id":"piafpookrbgwfmhcm1dao9x8te","creator_id":"d8njkgw3yifzjee3bejpkqb8kw"}
{"timestamp":"2024-04-29 14:57:26.872 -04:00","level":"debug","msg":"Unable to get a file reader.","caller":"web/context.go:111","path":"/api/v4/files/twq74tqr8bdzzxzthxzgfn6kqo/thumbnail","request_id":"za8bydua838adryqweinhzhmzo","ip_addr":"127.0.0.1","user_id":"d8njkgw3yifzjee3bejpkqb8kw","method":"GET","err_where":"FileReader","http_code":404,"error":"FileReader: Unable to get a file reader., unable to open file 20240429/teams/noteam/channels/piafpookrbgwfmhcm1dao9x8te/users/d8njkgw3yifzjee3bejpkqb8kw/twq74tqr8bdzzxzthxzgfn6kqo/panic_thumb.jpg: open data/20240429/teams/noteam/channels/piafpookrbgwfmhcm1dao9x8te/users/d8njkgw3yifzjee3bejpkqb8kw/twq74tqr8bdzzxzthxzgfn6kqo/panic_thumb.jpg: no such file or directory"}

Expected: If possible, a static preview image will be generated for the image

Note: The Go library for WebP files doesn't fully support animated webp files which may make this difficult. If that's not possible, we may have to look at not even generating an image and falling back to treating the file as if it were not an image at all.

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JIRA: https://mattermost.atlassian.net/browse/MM-57742

[arman-yekkehkhani]

Hi, I am interested to contribute. Can I work on this one?