While I can understand the difficulty of the drag and drop part, I think the comment on using shift or ctrl clicks to select multiple files should be much easier to implement, and solve most of the problem. Most programs that do this have text in the file line that looks like this: "1.jpg" "2.jpg" "3.jpg" and so on, if that gives you any clues on how to code it. I can try to look for programs that do this if it might give some inspiration.
Bug #:
Bug 7561