What is the code to manually insert the url extension for a file named using the option for Detail Pages -> Generated File Names -> Serialized Names.

Something like this but for serialized numbers. Ignore the Field_001 variable, it's just an example.

../[WM-Field: Field_001 nolink].php">

The only option I see is to rename the productID of every record in ascending order. Too time consuming and one more step that should not be necessary.