Marketplace: There's no message about an unaccepted file format

Steps:

1. Go to the Marketplace
2. Log in as a Connect subscriber
3. Scroll down and click “Add your package to the Marketplace”
4. Choose a file of a different format than .zip

Expected result:

 It's impossible to add the file or the user is informed that the format is wrong.

Actual result:

The user can add the file to the form, no message about a wrong format is shown (see screenshot).