Sorry to be annoying, here is part of nginx config:

# Upload form should be submitted to this location
location /upload_test {
upload_pass @test;
upload_store /usr/local/apps/Opus/temp/upload 1;
upload_store_access user:r;
upload_set_form_field $upload_field_name.name "$upload_file_name";
upload_set_form_field $upload_field_name.content_type "$upload_content_type";
upload_set_form_field $upload_field_name.path "$upload_tmp_path";
upload_aggregate_form_field "$upload_field_name.md5" "$upload_file_md5";
upload_aggregate_form_field "$upload_field_name.size" "$upload_file_size";
upload_pass_form_field "^(.*)";
#upload_cleanup 400 404 499 500-505;
upload_cleanup 404;
}

# Pass altered request body to a backend
location @test {
proxy_pass http://localhost:8080;
}

in case with firefox upload as you could see above, only "upfile" parameter is available, while upfile.path, upfile.name and others are missing.
That could not be related to backend as I used the same backend while uploading with the same form under Chrome and Firefox in above example.
Thank you.