Bug 1896901 - Don't use an <input> for unknown content type file....

Bug 1896901 - Don't use an <input> for unknown content type file. r=dao,desktop-theme-reviewers a=RyanVM

This happens because the input minimum size doesn't depend on the value
text. There's an experimental CSS property to support something like
this, field-sizing:

  https://developer.mozilla.org/en-US/docs/Web/CSS/field-sizing

Which is tracked on bug 1832409, but which we don't implement yet.

This makes the form minimum size account for the content, while keeping
text selectability. It's not ideal because there's no caret / Ctrl+A
etc, but it's probably an ok trade-off? Alternatively we can wontfix /
make this depend on implementing field-sizing.

Differential Revision: https://phabricator.services.mozilla.com/D210513