Enable submit button only when file is uploaded

Question

I have a form in which I have <input type="file"/> like so,

https://code.sololearn.com/W6Eo0HXn0pGT/#html

I've tried using "fileForm.$invalid" as suggested here: https://stackoverflow.com/questions/37016246/

But, it doesn't worked out. However, it seems (from reading comments) like it worked for the asker. What I am doing wrong?

Answer 1

https://code.sololearn.com/W9hG1Ki38aVF/?ref=app

Answer 2

Set the Button type to submit
EDIT:
And as well the file box to required.