I’m not sure why they chose to restrict the file attachment feature in the G1’s built in GMail application, but apparently it only lets you email image files as attachments FROM your G1 phone. However, you can rename a file with the the extension .jpg and it will show up in your file attachment feature suitable for transmission. When it arrives at its end destination, simply change the extension back and you should be in business.
Tested with .zip and .csv files so far. You may wish to use a free file manager app like Linda File Manager (from the Android Market) to help facilitate this work around.