Try microsoft outlook

Sounds very much like a MicroSoft error message: technically correct and utterly useless.

Is this problem limited to Word documents (.doc or .docx)? In other words, you can go through the same steps as other kinds of files you want to attach and at the last moment, nothing shows up in your message? Or is there an error messages from Google? What happens exactly?

The problem resolved itself. I tried restarting Firefox but that did not help. There was no error message. The file attachment process simply never completed. Very odd. Thank you for your polite response.