Revert "Fix bug DNN-4658: rollback changes made in FileManager.cs for issue DNN-2949, and if the file is not allowed in current instance and the user is host user, we need temporary allow this extension and log warning message in event log, after file added, we need remove this extension immediately."

This reverts commit 5c433d849e4f28887d79f5e4dfd370d80b6b36a4.