I’m testing the import of files in a cloud environment and I find that an error is simply generated without notification in the log. I’ve noticed that if the file exceeds 1.2 MB it just does not go up. In my local environment this process works without any problem, I have validated permits and everything is as it should be.
Google for nginx file size limit. You probably have standard value for this attribute.
Also you can find respond code for your request in browser console (shift+ctrl+i -> esc) with can be useful if it is not about file size limited by nginx, but likely it is still about nginx config.