All that header does is tell the browser to treat the response data as a downloadable file with a certain name. It doesn't actually set any response data which is why it's blank.
You'd need to set the file contents on the response for it to work.
@app.route("/<file_name>")
def getFile(file_name):
headers = {"Content-Disposition": "attachment; filename=%s" % file_name}
with open(file_name, 'r') as f:
body = f.read()
return make_response((body, headers))
EDIT - Cleaned up code a little based on api docs