every client should do that but that's not a solution that works. there are too many clients that don't care.

Additionally, who determines what size is ok and what is not? If you have plenty of bandwidth 50MB might even be acceptable whereas in some cases 50KB might be pushing the limits already, so any solution can only be on the receiving end i think. If you do not want to waste large amounts of bandwidth the receiver needs controls for that. Either block it or do something like @ᴛʜᴇ ᴅᴇᴀᴛʜ ᴏꜰ ᴍʟᴇᴋᴜ is suggesting. I would settle for simply not showing and use a placeholder or something.

often you don't know how large a file is before you download it