Automatic File Extension in BrowseForSave

Question

Is there a way to set the extension of a file using
flash.FileSystems, browseForSave function?
The browseForOpen function takes a FileFilter. However when
using browseForSave you can not set a fileFilter and the file
extension has to be typed in manually. The File.extension property
associated with the selected file is read-only. Does anyone know of
a work around or anything that I could do to not enter the file
Extension manually.

Answer 1

For others if they would like to see:
set audEXT to {"mp4", "m4a", "mp3", "aif", "aiff", "wav", "m4r"}
property FoldersToMake : {"AUDIO", "VIDEOS", "Python workspace", "Documents", "PHOTOS", "Quartz Compositions", "Other"}
          tell application "Finder"
       repeat with newFolder in FoldersToMake
                                        try
                                                       make new folder of folder "CleanUP" with properties {name:"" & newFolder & ""}
                                        end try
                         end repeat
          end tell
          move (every file of desktop whose name extension is in audEXT) to the folder "AUDIO" of folder "Stuff"