Is the Google GMail client able to download attachments for arbitrary file types?
As far as I know this was a new addition to the app recently, so it's surprising that it doesn't work for you.  I just tried emailing myself a .epub file and it worked just fine, I had a Download button to the right of the paperclip icon and attachment name.

I know that the Browser on many devices won't let you download any files that don't have a handler registered for their extension; perhaps this is related.  You can solve that issue via an app that registers for all files, such as Download Crutch; perhaps it will solve your attachment issues as well.  I'd reboot after installing it.  I have Download Crutch installed and have had no problems with it.