It's usually an EASY FIX.
It can bee that you are trying to attach TOO LARGE a file, in which case you can divide up your file and attach mulitiple files instead of one LARGE FILE.
However, most of the time it is because you are trying to attach a file that is is NOT allowed.
When you submit a ticket, at the bottom of your ticket you can see what ARE the Allowed File Extensions: Look at the image below:

Watch the video below for more information: