plain response:
curl -4 -F "fileToUpload=@/tmp/test.txt" -F "output=plain" -F "expire=86400" -F "maxhits=0" https://up.ovpn.to/upload.php

json response:
curl -4 -F "fileToUpload=@/tmp/test.txt" -F "output=json" -F "expire=86400" -F "maxhits=0" https://up.ovpn.to/upload.php