Hi @Tim Molyneux, the above post should definitely solve your problem (thank you @mikeb) as you are trying to load in a fixture personality file in the way of loading a fixture library file onto the desk. Using 'Assign Fixtures' -> USB as described in the manual above will work.
Regarding this, I thought I'd just chip in and mention about file formats for future reference:
JOS files are the file format for the Jester operating system when updating the console's OS.
JSF files are showfiles for Jesters in the 12/24 and 24/48 range.
JML files are showfiles for Jesters in the ML range, TL and TLextra range.
I hope this is of use for the future for file format knowledge.
Edit: To add, I have attempted to upload the IFT file provided above on my own JesterML console using the method in the manual via 'Assign Fixtures' and it does load in correctly.