Summary:

Fix for bug #1440: Web validator allows non-zip files to be uploaded

Expected behavior:

The result should be the same as when the Validate button is clicked when no file was selected. A message should say Please include a file to validate.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with gradle test to make sure you didn't break anything
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)