Preview of uploaded BMP image doesn't work (image opens fine)

This issue has been migrated from #15990.

Originally reported at vector-im/element-meta#1916, but this seems to be a server issue.

Example image: https://people.math.sc.edu/Burkardt/data/bmp/blackbuck.bmp

An encrypted room just shows an empty box:

The /thumbnail endpoint returns (https://matrix-client.matrix.org/_matrix/media/r0/thumbnail/matrix.org/UOupUIqZCoSRCUJTmNLthUgX?width=800&height=600&method=scale):

{
    "errcode":"M_UNKNOWN",
    "error":"Cannot find any thumbnails for the requested media ([b'matrix.org', b'UOupUIqZCoSRCUJTmNLthUgX']). This might mean the media is not a supported_media_format=(image/jpeg, image/jpg, image/webp, image/gif, image/png) or that thumbnailing failed for some other reason. (Dynamic thumbnails are disabled on this server.)"
}

Start at the /thumbnail endpoint and reproduce the failure with the linked blackbuck.bmp image, checking how the listed supported media formats are handled. Done means the endpoint returns a usable thumbnail for BMP uploads and the preview no longer shows an empty box.