[sda] User uploaded file sizes in files api

[pontus]

I think (#360 (comment)) it might be very beneficial for users to see uploaded file sizes in the files API.

However, the s3 proxy doesn't store that, and it seems maybe it's non-trivial to add that functionality (by a quick look, it appears s3proxy would need to track a bit what passes through).

I do see it as a fairly valuable feature for end-users, though - to give the comfort of seeing the size on our side matches their local size.

[jbygdell]

Once the file is uploaded the s3inbox can query the S3 backend for the file metadata, that would give us the size as well as the MD5 checksum (SHA256 is not available)