Error Message Uploading Image in Form

I’m running ‘Single App Edition’ and using the (I think it’s called) ‘Apps Company Previewer’ to view my work.

I’ve created a form with one of the field types set to image.  When I press SEND to upload my image, I get an error message “An Unknown Error Occurred”.

Not sure if it’s because the App is not published and I am using the ‘Previewer’ or if it should work.