file: Don't rely on application-x-generic
application-x-generic
is our preferred fallback themed icon. However,
it's not present in the icon naming specification[0] and, as such, is
not included in the icon themes some downstreams adopt.
Instead of immediately resorting to the unthemed fallback texture, we can give a second themed alternative which is likely available in any icon theme: text-x-generic.
[0] https://specifications.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
Fixes #3268