"I think you've got a great idea and a great product here because I love my Gmail and I've always wanted the power of email client with a webmail account."
Solution: Recreate your Gmail session by clicking the Mailplane > Reset Gmail Session menu item.
“Attach file” link: Only one file at the time can be uploaded
The reason for the change: Gmail frequently got a white screen because of the Flash movie required for the "advanced attachment" feature. I had no choice but to disable this flash movie.
As soon as the white/blank screen is fixed by Google, it'll be reenabled of course.
Next update will add the multiple file select feature back.
Meanwhile, please try the following workaround to get the multiple file upload back:
Use drag and drop: In Finder/iPhoto etc. select multiple files and drag them to the Mailplane window to attach them all at once. You can also drag files to the Mailplane dock icon to compose a new message.
When composing rich text email messages: No spell checker items in context menu
Workaround: Double click word before opening the context menu or switch to 'Plain text' emails.