This seems odd. If it wasn't meant for Nginx to process uploaded files, then it couldn't have processed them by itself.
For example, I changed
location @test {
proxy_pass http://example.com:8080;

to
location @test {
default_type text/html;
root /www;
and removed the :8080 server part and it still works.
So I am inclined to think that there must be a way to set the filename without going to such lenghts as to change the code.