Error : can't leave Input file empty

Hello everyone,

I would like to set up a 'file upload' where you can upload a file, and keep it in a 'formOutput.fileStudentPlanContract' (with fileStudentPlanContract being a contract of a file type).

I can later retrieve it with no problem, however I can't seem to leave the field empty, (as if it was mandatory) as this debug msg is displayed :

Debug message
Error while validating expected inputs