Web app integration preliminary callback

How am I supposed to handle the preliminary callback containing the file? I am trying to use a url that is making a request to my server, but nothing happens I get the message "Sorry, your action failed to execute. Please try again."

the same url works if I use it as client callback url. I also tried to set https:// requestbin as a callback url and it works.

in the documentation there is something mentioned about "If the application specifies a file parameter in the REST method, then the preliminary callback URL cannot originate from the client. As a result, a second request must be made from the client to your server so the server can send the necessary interface to the user." I am uncertain of what this means?