hey there @Giulico ,
it seems like the acceptable file upload size may have changed, and we have not yet had a chance to update our documentation. Currently, the whole request body needs to be less than 8mb in total size, and it seems like everything over that limit causes problems.
We are going to update our docs, but in the mean time, we encourage you to make it clear to people using the form that the file size needs to be smaller than what it currently is.