We thumbnail and serve emoji with the same format as they were
uploaded. However, we preserved the original extension, which might
mismatch with the provided content-type.
Limit the content-type to a subset which is both (a) an image format
we can thumbnail, and (b) a media format which is widely-enough
supported that we are willing to provide it to all browsers. This
prevents uploading a `.tiff` emoji, for instance.
Based on this limited content-type, we then reverse to find the
reasonable extension to use when storing it. This is particularly
important because the local file storage uses the file extension to
choose what content-type to re-serve the emoji as.
This does nothing for existing emoji, which may have odd or missing
file extensions.
Hash the salt, user-id, and now avatar version into the filename.
This allows the URL contents to be immutable, and thus to be marked as
immutable and cacheable. Since avatars are served unauthenticated,
hashing with a server-side salt makes the current and past avatars not
enumerable.
This requires plumbing the current (or future) avatar version through
various parts of the upload process.
Since this already requires a full migration of current avatars, also
take the opportunity to fix the missing `.png` on S3 uploads (#12852).
We switch from SHA-1 to SHA-256, but truncate it such that avatar URL
data does not substantially increase in size.
Fixes: #12852.
Actions like deleting realms may leave unreferenced uploads in the
attachment storage backend.
Fix these by walking the complete contents of the attachment storage
backend, and removing files which are no longer present in the
database. This may take quite some time, as it is necessarily O(n) in
the number of files uploaded to the system.
Alex Vandiver
Anders Kaseorg
AcKindle3