FileMask doesn't affect Upload Dialog

I use the ImageUploaderFlash on a web site to let users upload one single PDF file. The following works fine for the most part. When users try to drag/drop any other file type or more than 1 file, it lets me give them a warning

<Restrictions FileMask= "'*.pdf' MaxFileCount="1" />

The problem is, in addition to dragging/dropping files I enabled the button that lets users open a windows file dialog to search for their files , the file types dropdown automatically defaults to image files and gives you the option for "Images, HTML Files and all files.

Is there a way to make pdf files the only type of document that shows up in this dialog? I've tried

FileMask=
"[['Documents (*.pdf)', '*.pdf']]"

But that still doesn't do anything either. I'm trying to make this site as dumbed down as possible and we instruct users to only upload a PDF, but as any web designer knows, you have to try to make your interface as simple and half-holding as possible. If people are allowed to try to select file types that they can't use, they're going to get frustrated.

Here is a screenshot to help illustrate my issue:

www.bulala.com/filemask.png