Geary should have an accelerator for the "Attach File" action.  My 
recommendation is Ctrl+T, for aTtach, since A (select all) and F (find) 
are already taken.

This patch should work, but for some reason, it doesn't.  It's not that 
the accelerator is being used somewhere else -- we can change the key 
combination and nothing happens.

Note that we also need to update the documenation, which this patch does 
not do.