How to reproduce:
Open email with attachment, click on attachment in order to open it.

Actual results:
Attachment is automatically opened/shown in new tab.

Expected results:
User should be asked if attachment is supposed to be opened in new window or new tab or whether it should directly be saved/downloaded.

This could be implemented by adding an extra step where (by clicking on an attachment) a window pops up, offering the following 4 options (see screenshot for an idea):
Open in new window
Open in new tab
Open (with ... [program])
Save

Options 1 and 2 open the attachments in the email program (like it also happens now), option 3 opens the attachment with the corresponding program on the computer (e.g. Adobe Acrobat Reader, PowerPoint, Word etc.) and option 4 saves/downloads the attachment without opening it.

The main issue here ist to let the user select what he/she wants to do!