Theopenem doesn't expose files directly. It needs to go through an api, meaning you can't just put a file on there and access it through a url.
An example for wimboot would be
http://192.168.56.1:8088/clientimaging/IpxeBoot?filename=wimboot&type=bootimage
This would require the wimboot file to go in the pxeboot images folder.
A better way would be just to start up another web server that can serve the files directly. You could just use another port if you want to stay on theopenem server. Then theopenem would only handle transferring the initial pxe file over tftp, the rest would be up to you.