Uploader, multiple file selection, resolves #1279

This fixes #1279. See the mentioned issue for a clear demonstration of this bug.

On Safari/Windows, you cannot select multiple files when trying to upload. However, if you select only a single file, it uploads properly.

Unfortunately, feature testing the file selection feature of the `File` API seems impossible, as we simply do not have access to the user's file system. As a result, user agent detection must be used.