Kevin,

Good call. Our hands are a little tied with this, in that we only have a handful of standard file selection dialogs to choose between. We opted for the one that offers a Preview of the file chosen, feeling that matched better with the Attach dialog. We could go with a larger and resizable selection dialog, but it would not have the Preview.
However! In revisiting the possible options, I found that one offers a preview pane, with the caveat that the dialog does allow selecting more than one file. We'll take a look at this and see if we should swap the dialog out.

--J