Mailplane has always had multiple ways to attach your files and pictures to a message: Drag and Drop, iPhoto plugin, cut and paste, the media browser, screenshots, PDF Service, and more. Now Google has added something many users have waited for: Progress bars when uploading attachments.
The new "uploader" is based on the Flash plugin. Some users may experience problems using it. You can always revert to the "classic" attachment uploader if you have trouble. A new attachment setting in your Gmail settings let's you switch: