Can't attach attachments

In Mail (Im using a .mac email address): when I drag a file to attach, or use the 'attach' button inside a mail message, nothing happens, the message does not get a file attached to it, it just stays blank or with whatever was there before. If I go to me.com and log in, every time I can attach the same file no problem.

It has happened to any kind of file, when it happens for example with a jpeg, from then on, no file no matter what it is will attach. I am aware of size/file restrictions, that is not the problem.

This is a strange phenomenon that happens randomly. Usually when it starts to happen it will do it 1-2 days long and suddenly without anything specific steps will work normally again. Restarting the computer helps only sometimes.

This is something that has been bugging me for a year or so now and on 3 different computers. I cant remember if this started to happen after upgrading to 10.5 or it happened before as well. I cant remember anything on any of the computers that changed that might cause this.

I always assumed this was a (very annoying) quirk of Mail, but finally decided to ask someone about it, but I don't see any forums on this topic, I guess It's not that common?

Any ideas? thanks