Populate list of filetypes in Application.GetOpenFilename method.
This command will have multiple options for user to pick from filetype dropdown when asking user to select a file to use (open or save since this also works for Application.GetSaveAsFilename too)
LD1 = Application.GetOpenFilename ("All files,*.*, All images,*.jpg; *.gif; *.png, LDraw files,*.ldr;*.mpd")
Will list it as
All files (*.*)
All images (*.jpg, *.gif, *.png)
LDraw files (*.ldr, *.mpd)