dragData["files"] question

[FlexibleLearning]

Is it my syntax, or is it simply not possible to modify the
dragData["files"] during a file drag operation?

If I drag a .png file and a .rev file, I would expect the .rev filepath to
have been removed, but it isn't...

on dragEnter
  put the dragData["files"] into tFilePaths
  set the itemDel to "."
  repeat for each line L in tFilePaths
    if (last item of L is NOT among the items of "GIF.JPG.JPEG.PNG")
    then delete line lineOffset(CR&L&CR,CR&tFilePaths&CR) of tFilePaths
  end repeat
  set the dragData["files"] to tFilePaths
  put the dragData["files"] --> Value does not get updated!
end dragEnter

So what am I missing?

Hugh Senior
FLCo