I have actually tested that and for some reason, Anvil recognizes .csv files as ‘application/vnd.ms-excel’. However, this is not the problem, as it does not even execute the first line of the background task where I set the state[‘index’] to 5.
Here is a copy of my application.
https://anvil.works/build#clone:FCCQXIGBTQEODQIJ=WM2YZAYIF3VMQCAZ5BRR4DVO
It is somewhat big, so the code that you are looking for is in the ‘file_service’ server module and in the MatchForm where I have a timer for testing and an upload button.