How odd, I just did a test and e-mailed a tif file to both my Gmail account or pop3 account.
With the Gmail message it is previewed in the message(I attached it originally) and when I hit the preview button it will open it up and display it in the gallery and I can view, zoom it and move it around (nothing else but set as contact icon or wallpaper).
With the message in my pop3 account I can't preview it, but I can save the file to the SD card. Once saved I can tap it in Astro and it asks to open in Gallery or Camera or Astro Viewer but it doesn't open in any of those.
So it looks like the only way is to e-mail it to your Gmail account and you can open it with ""preview" button (I used the Gallery option) and view/zoom it from there.