From experience we know by now, that it is absolutely important not to close the browser or the site tab before a file upload is completed. Otherwise the post will contain a zero kb file. T Yet this file is named correctly, so you might not recognize the failed upload immediately.

The upload time depends on file size and on the general speed of your internet connection. To be on the save side, you should wait until you are automatically redirected to the forum.

By the way: Don't forget to put a file placeholder in the Message box before operating the Submit button.