Tested on latest Chromium / Firefox. In case of .m4a files they want audio/x-m4a and not audio/m4a, in case of .flac files they want audio/flac and not audio/x-flac. The module we useed to detect the types however detects them only as audio/x-m4a and audio/x-flac as they are not offical IANA supported mime types (not in IANA spec == "x-" prefix): https://www.iana.org/assignments/media-types/media-types.xhtml Though flac is not in the IANA spec many programs such as the file command (https://man7.org/linux/man-pages/man1/file.1.html) and Chromium (flac) / Firefox (x-flac and flac) support audio/flac only or both.
We need to remove the metadata without breaking the animation.
For that we use sharp which incooperates libvips (binaries for most common distros included).
This also decreases client side upload complexity as we remove the metadata on the serverside.
Sharp: https://sharp.pixelplumbing.com/
libvips: https://libvips.github.io/libvips/
By default we take the slug given in the request, if this is not set we try to give a filename from known types.
If we still have no filename we fallback to the previous method of setting no filename.
If the filename is non ascii we will only create the encoded "filename*" and not the ascii only "filename". This is to prevent other applications to save a file like "?????.png" if the filename contains non ascii chars.
For the browsers nothing will really change comapred to the behaviour before this change as good fallbacks if no content-disposition filename is set. But that is not the case for all application, thus it makes sense to include the proper way to set the filename.
