Instruct stashed upload failures to reupload file

Instruct stashed upload failures to reupload file

Given enough permissions, anonymous users can upload files
just fine. But when there are warnings, their failes can't
be stashed.
This is handled correctly by adding the <input type="file">
back to the form, but that could easily be overlooked:
- if you're used to being logged in and having your upload stashed
- the instructions only mention changing the description

This will slightly alter the instructions in such case so
that users are directed to re-upload their file when it
couldn't be stashed.

Bug: T115822
Change-Id: I81f5b112d158fe5f8b0850654af07d248bc3fda5