As a regular user of Thunderbird, I receive emails from my bank that contain payment confirmation documents as PDF attachments. Now, to further process each attachment (forward, save, etc.) I need to click on its icon and wait till the preview loads in a separate window. Then I need to close the window, before I can proceed with the processing, remembering its contents first.
I propose a feature that allows for a automatic preview of PDF attachments directly within the email message window, either at the end of the message preview pane, or in a sidebar for wide screens.
I guess that this feature should be limited to emails received from trusted senders. For me, a sender can be assumed trusted if they are included in my address book.