Allow update of existing project document

Prior to this, it was impossible to update a document's title or
description without uploading a new file, as MantisBT would trigger an
error.

Fixes #17405
Add error param when updating project doc with empty title

Added missing error_parameter() call so that when attempting to update
an existing project document with an empty Title, the error message
indicates which field is required.

Fixes #17407
Remove extract() call on untrusted input

As per recommendation in PHP manual http://php.net/extract, drop use of
extract() on input from $_FILE superglobal.