Microsoft Word has special characters that can impact the Webmail e-mail client's handling of attachments. For example, typing "..." in a Word document produces a single character. If that single character is pasted into a Webmail compose window and a file is attached to the message, the attempt to send the message will fail.
The solution to this problem is to not copy/cut and paste from any word processor documents. If you wish to type something up before actually sending it, use a basic text file editor (Notepad, vi, etc.).