That's not really anything unique to this. You would have the same problem with any browser extension or userscript. This isn't even a problem unique to the browser. Client side validation is a hard to enforce concept. However, being able to supply your own files is hardly a vulnerability.
For further actions, you may consider blocking this person and/or reporting abuse
We're a place where coders share, stay up-to-date and grow their careers.
That's not really anything unique to this. You would have the same problem with any browser extension or userscript. This isn't even a problem unique to the browser. Client side validation is a hard to enforce concept. However, being able to supply your own files is hardly a vulnerability.