We implemented multiple uploads in [#4659], but it's not really clear how to use it. Also, the HTML5 method can be a pain if you're trying to attach multiple files in different directories or stuff like that.
One possibility is to spawn an additional "Browse Files" after selecting a file, like is done on SourceForge File Release System uploads.
While we're at it, we should also add the ability to "cancel" an attachment selection. Currently, there doesn't appear to be a way to do that.