Test the behavior of the file-storage when a malicious user would try to store a pre-existing file on the server as its own
The fix for the file-storage is a simple validation to make sure that the tmpfile exists, however, for the generic case of the file widget, we cannot trust the tmpfile value when this was not generated by the server. This will probably cause regression when one wants to show a "preview" of a form, to be continued.
Make use of util::file_content_check and check also in other cases
This change also covers the case, where the checkmark for uploading zip files was added marked in "upload file". It will also report errors which were silently swalled before.