Upload form on mobile

[rebeccacremona]

On mobile, input#filename escapes its container and the overflow is cut off. As well, since button[type="submit"].is-hidden-mobile, so far as I can tell, there is no way to submit the form from a mobile device, unless that device is using an external keyboard.

I spent a few minutes trying to fix while working on the aforementioned accessibility PR, but it was proving sufficiently finicky that I decided to make an issue instead. Might take another swing at it in a bit.

[ikreymer]

Thanks for reporting! The overflow cut-off should be fixed now. The field should submit on pressing enter on a mobile keyboard even without the button visible, but do you think that's not clear enough and should still show the button below?

[Shrinks99]

Fixed in #295