Feature suggestion: Save attachments from the main window.
In the main window, we can delete attachments but can't save attachments.
I suggest add a command to the "Mail" menu and the context menu of Mail List in the Main Window to open the "Save Attachments" dialog of the selected mail. And download the attachments if they are not downloaded yet.
Further improvement (but not necessary):
When multiple mails are selected and filenames of attachments collide, auto renaming or allow renaming attachments in the "Save Attachments" dialog.