process big attachments on a background thread
1. when i click on an email with a big excel attachment (7MB, 23000 rows) Airmail shows a beachball cursor and becomes unresponsive for ca 30-40 seconds. after the mail is opened the attachment displayed as an icon.
2. when i doubleclick the same email, a small preview window is opened and airmail freezes
idea: don't process big attachments on the main thread