upgrade from MF-TZ21 to 7 Speed cassette freehub I'm trying to save costs so I want to upgrade to 7 speed 11-28 Cassette to keep the same shifters, dealer says I need a new Freehub, new 7 Speed Cassette, new chain AND NEW DISC BRAKES, why do I need new Disc Brakes? I already have them.

UPDATE: researching I've found it is a 6 Bolt Disc Brake (I added a new photo) so I think seller was trying to sell me what he had (Centerlock hub I think), not what I need (36H 6-Bolt rear hub).
Pretty sure that the MF series has the freewheel integrated with the cassette, so if you are going to a "pure" cassette, you will need to replace the freewheel and possibly the hub.  You would need to replace the hub if you can't find a conversion to a non-integrated freewheel for your current hub.
If the hub gets replaced, that could affect the disk as well.  Does your current disk have an ISO 6 bolt pattern, or something else like Shimano's centerlock?  If it's an ISO 6 bolt, you should not need a new disk with a new hub, assuming you get a hub with an ISO 6 bolt pattern.