Do you want the desktop app to do this automatically? If that's not required, you could just make a web form that has a file-type input field, and point the form's action to a script on that server that will accept the file and process it as desired.
If you do want the desktop app to handle it, then it depends on what functionality is available to that app's programming language/environment. If it can process cURL requests, that would be a fairly simple way to submit the file in the same manner as the HTML form described above would do it.
