A little more experimentation with SWFMill to peek at the flash movie showed the use of the FileReference and the FileReferenceList Object. A simple FileReferenceList.browse() opens up the multi-select file open dialog. After selecting multiple files, the file names are passed to the FileReference Object as an array.
Interestingly, I looked at the Web Forms 2.0 and discovered that this feature could be easily implemented using the max and min attributes that would be supplied to the input type. However, this is still a working draft, and could take some time till it is pushed into HTML 5. Here is how Web Forms 2.0 map to the HTML versions.