Mac Mail won't attach some files
Suddenly my Mac won't attach files to emails. Not every file, some still work, but most. When I drag the icon of the file to attach it, it says "downloading" beneath it. (This is to attach a file to send it, not to download an incoming file). I have no idea why this started, but I updated the system hoping it would clear it, and no luck. I can send the email, and it shows an attachment on the receiving end, but it either can't be downloaded or says corrupted. I'm stuck. Thanks in advance. OSX 13.5.
Mac Pro, OS X 10.11