There has been some issues with Signal attachment service providers - there are two currently - and during my tests this weekend I have always gotten the one and same which works. There may be something wrong in our code as well, or it could be both.
We don’t have any logs of the issue yet, which makes fixing it tricky, to say the least. So, if someone happens to catch logs of a failing attachment upload, please send it to us! (And don’t forget to sanitise the log before posting it publicly)!