Idattribute must be a variable in the VariableValue database. Are you simply looking for THE file name? If so, that's your issue. If not, you need a variable name "Filename" and a file with that full value (including the extension) must exist.
As always Jeff, spot on. Since I am importing the models from STEP files they are empty of variables. However, I can run another scheduled task to add a variable for the part "Number" and now key off that for matching the remaining properties in an XML import.