Attachment Extension Blocklist in Dialogs

Reverts the tBuiltInPermList behavior to that of the pre 21.011.20029 build. Beginning with build 21.011.20029, extensions added in the tBuiltInPermList with restriction level 3 will be missing from the menu items in all the save and open dialogs. This feature is enabled by default. Users can revert to the old behavior by setting FeatureLockDown/bEnableBlacklistForOpenSave to 0. Possible values include: 0: Revert to the legacy behavior. 1: Do nothing. Retain the new behavior.