There is discussion on this going back years on the Google forums I see. One posted had a semi workaround:
As other posters have stated, temp files are created for downloads, but you don't have to download every attachment every time. You only have to download each file type ONCE before being able to choose to open it, rather than download.
E.g. Download a *.ppt file and then in the download box, click the 'down' arrow and choose "Always Open files of this type" and the next time you download a *.ppt file, it will open with your default PowerPoint program, WITHOUT downloading it other than to a temporary file. They won't then fill up your Downloads or any other hard drive folders
If you change your mind later, go to the 'Customize and Control ...' icon and select OPTIONS/UNDER THE HOOD/SECURITY and SELECT clear the settings. Unfortunately, this clears all the 'auto-open' permissions.
The drawback being that the files stay in your download folder. Another poster mentioned he then adds that folder to CCleaner, and has it clean it. Source