Thanks for reporting this bug.
We found the issue and will release a fix for it today.  


Issue:
When adding a map or mod without fully completing the steps, it will remember the step you left off on. The next time you go to upload a map or mod, it will resume from where you left off. 


Manual Solution: (temporary)
Press the red 'Cancel' button located in the bottom right of the steps.  This will fully cancel the submission process and you can start from step 1 again.


Hope this helps!

This bug should now be resolved. 
Clicking add map/mod will reset the submission process automatically.

Let us know if you have any further issues.