Sorry for my English 
Some days ago I found what router drop download via browser. For a test I change MTU, MRU and MSS via Mangle, but nothing work. What can I do for fix this problem? P2P work perfectly.
;;; RouterOS v6.19
;;; IP -> DHCP Client -> ether1
Add Default Route: special classless
Default Route Distance: 10
;;; PPP -> Profiles -> Add New
Name: PROFILE-ISP
Remote Address: 127.0.1.0
Change TCP MSS: no
Use MPLS: no
Use Compression: no
Use VJ Compression: no
Use Encryption: no
Only One: yes
;;; PPP -> Interface -> Add New -> Client L2TP
Name: L2TP-ISP
Max MTU: 1420
Max MRU: 1420
Connect To: vpn.rsv56.ru
User: xxx
Password: xxx
Profile: PROFILE-ISP
Keepalive Timeout: 60
Add Default Route: [x]
Default Route Distance: 1
;;; IP -> Firewall -> NAT -> Add New
Chain: srcnat
Out. Interface: L2TP-ISP
Action: masquerade
;;; IP -> Firewall -> Mangle -> Add New
Chain: Forward
Protocol: TCP
In. Interface: L2TP-ISP
TCP MSS: 1421-65535
TCP Flags: syn
Action: change MSS
New TCP MSS: 1420
;;; IP -> Firewall -> Mangle -> Add New
Chain: Forward
Protocol: TCP
Out. Interface: L2TP-ISP
TCP MSS: 1421-65535
TCP Flags: syn
Action: change MSS
New TCP MSS: 1420