Try going into settings from somewhere in Gmail, choose the General tab, and at the bottom of the page you should see
Quote:
Attachments:
Advanced attachment features - See progress bars when attaching files to messages and attach multiple files at once. Requires flash. Learn more
Basic attachment features - Attach one file at a time and don't show progress bars.
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with a radio button to choose one option or the other. Make sure Advanced features is selected.