We just installed Google Apps for Domain at work, and some of our employees are replacing Outlook with Gmail. One of the complaints was that there was no way to drag attachments from a folder into a Gmail message. I struggled to find a solution to this, and eventually came across and incredible Firefox Add-on that does exactly that:
Simply drag the file from your desktop or any folder, into the attach a file link in Gmail. This works for both regular Gmail, and Gmail for Domains.