First, GetData method (this is function) REQUIRES FORMAT and RETURNS DATA, i.e.:
DataVariableName = GetData(2) && 2 is for bitmap

Second, files must be retrieved by Files property:

*-- Method TreeView.OLEDragDrop
LPARAMETERS data, effect, button, shift, x, y
Private FilesCount

IF data.GetFormat(15)
FilesCount = data.files.count
DIMENSION laFiles(FilesCount)
FOR i=1 TO FilesCount
laFiles(i) = data.files(i)
wait window laFiles(i) timeout 2
ENDFOR
ENDIF