Can you please try to re-save your workflow and upload the new copy? I have been unable to replicate the error, and based on the message you provided it may be misreading the file type.
If you are still unable to upload please let me know so I can troubleshoot further.
@JDelgado, If you go into the Interface designer properties you can check the name of the browse tool that will display on success. In the solution posted you would check "Browse (69)" and when you run the app you will see the results pane display.
I had this question too and finally figured it out.
I want to post my solution, but when I try I get an error.
"The contents of the attachment doesn't match its file type."
It occurs when I try to attach ".yxwz" or ".yxmc" files.
btw ,it was great challenge!