We saw the same thing. It seems to be a function of the way the window is created when you compose a message.
We were able to fix it by:
In IMP click on "Options"
Click on "Message Composition" in the right hand column
Clear the check box that says "Compose messages in a separate window"
After doing this, the attachment button works properly for us.
I'm sure it's possible to change this so the box is unchecked by default (for all users) but I haven't tracked it down yet.
Hope this helps.
Robert