Bug in handling POST then sending a file back

[Saint Michael]

I am using openresty and nginx.
I send a file to the server, which is a POST operation.  Then the
server processes the file and needs to send back a different file. I
try to send a file with ng.exec("/static/file_name") and I get error
405 Not Allowed.
But if I do a 302 redirect, it works.
I imagine that Nginx feels bad about sending a file in a POST
operation, but http does not have such a limitation.
Is there a workaround for this?