In the Finder, select the icon of one of your files (the ones you refer to as "attachments") do a Get Info on the icon of that file (Command I) and select what application you want to open it with, then click on the Change All button. You'll have to do this once for each type of file you want to open with Photoshop, i.e. once for TIFF, once for a JPEG, one for a PSD, PSB, etc.
