Hi @Stockton
unfortunately this is known issue, it happens when test step and the file are larger than 4 MBs as these files are stored long term, while testing flow would store them locally and delete the files from our servers after run is completed.
There are two things we can do for sure.
figure out how we can optimize for higher files limit during testing and show better error message for sure.
Workaround: Test Flow would work but not Test Step unfortunately.