Fitnesse - 17.1 - File Upload Objects

After upgrading to 17.1 Appian (and have now also upgraded to the latest version of Fitnesse) we're having issues with a file upload section in our scripts.

Accessing the field using the old way (POPULATE FIELD | example field | with |..) no longer seems to work for our page.

I appear to be able to access (it's at least not failing due to not finding the field) the new upload field using the below statement (or just using index on page seems to pick it up as well) but it's throwing an error.

|populate |FILE_UPLOAD |field |[1] |with |C:\test.doc |

Error message: Invalid for a READONLY Field

Any clues on a possible solution?

Thanks in advance :)