If we are dealing with a normal file serving and not a range request, and it's sent as-is (eg binary and not using deflate / br etc) then we can sign the file with the sha1 hash which is already known ahead of time:

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Digest

Note: the point of this is not to just send a digest which we could do with nginx or apache and sign what was actually sent, the point is to mitigate a risk of the file storage layer has been corrupted or compromised so that what was sent is what we expected should have been sent.

It would be nice to also Subresource_Integrity but it doesn't support sha1 (see MDL-58083 / MDL-58066) and that would also be blocked by MDL-66006