I am bumping this thread because Google has changed the format of the GMail interface. Previously, my GMail interface had a few words printed in blue that said "Attach a file" or something like that. You would click on these words to get the file attachment function. But unfortunately for me, once I had attached one file, the file attachment function would no longer be available to attach additional files. (Yes, this is contrary to Blobbenstein's experience.)
Now that the interface has changed, it has an icon in the form of a paper clip attached to a sheet of paper. When I use this newly changed feature, I can just click it again to attach a second file, etc. So that problem has been solved.
The only thing I don't like about the new format is that in reply messages, it puts the quoted material in a kind of small rectangle with an ellipsis, and this is placed below your cursor. In other words, it encourages you to "top post." I was taught that top posting was wrong. I don't think Google should be encouraging this behavior.