I just ran a quick check on a file that I have open with fileMaker 11 and another that I have open with FileMaker 12. Get (FileName) returned identical results with both files--it returned the name of the file without the name of the file extension.
Thus, the issue does not appear to be with Get ( FileName )
Is the file from which you are importing data located in the same folder as the FMP 12 copy of your database? (From what little you have posted here, it looks like you are relying on FileMaker's relative path feature to find the file from which your script will import the data.)
If you changed your calculation to Get ( FileName ) & ".fmp12", does your script work?