Try this for a test:
Put the cursor in the last row of the portal. Then run your insert file script. Does it create a new record and insert the file without overwriting existing files?
If so, you may need to add just his one line to your insert file script:
Go To Portal Row [last]
If you have more than one portal on your layout, you may need to tell FileMaker which portal you are referring to when you tell it to go to the last portal row.
Use the Name box in Inspector to give your portal a name such as "File Portal".
In your script:
Go To Object ["File Portal"]
Go To Portal Row [Last]
Insert File [...