I have had a persistent problem for the last couple weeks where Mail.app won't properly add attachments when composing emails (note that receiving attachments seems fine). Dragging a file to a email compose window will result in an icon (typically, it would show an inline preview of the file if possible) with the correct filename but of size 0K. When using the attach command a selected file will just not appear in the compose window. It seems this error occurs after attempting an attachment in Console: Mar 29 ********** com.apple.security.pboxd: CGSReleaseWindow: Invalid window 18963 Mar 29 ********** com.apple.security.pboxd: _NXTermWindow: error releasing window (1000) I have deleted all Mail preferences as well as removed the appropriate "Mail Downloads" folder in the Containers directory and even the entire "com.apple.mail" directory. I have deleted all accounts and started from scratch. An alternate user results in the same issue. Interestingly, if I drag a potential attachment file to the ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads folder manually, then drag that file to an email, it will work. There seems to be something wrong with sandboxing and Mail.app for me. I have also used utilities like OnyX to verify permissions for my user directory. I know reinstalling OS X is an option but I'm on a legacy MacPro that's technically not 10.8 compatible without going through hoops and I would really, really hope to avoid that process for now.