I've had quite a few questions recently with attachment problems on Windows 8. The users either couldn't figure out how to close attachments or wanted to stop the attachment from opening in full screen. This is the result of opening the attachment in a Windows 8 app, not a desktop application. The solution: change the default program for that file type.
How do I open an Outlook attachment and with one keystroke close the attachment and return to Outlook? When I press Alt & Tab, it closes the attachment and I return to outlook, but the attachment is still open but hidden on the left side of the screen.
You're opening the attachments in the Windows 8 apps, not a desktop application, and need to use Alt+F4 to close the app, not Alt+Tab.
Change the default application for an attachment
To change the application that opens the attachment, click on the notification message in the right corner of the screen that tells you other programs can open the file.
Select a desktop application from the list.
If the notification message doesn't come up, save the attachment to My Documents, then right click on it and choose Choose default program... then select a desktop application. Future attachments of that type will open in the desktop application you selected.