Problem with blocking upload file TMG 2010

I'm using TMG 2010. I have 3 rules : 

1/Allow Internet Access : 

protocols : dns, http, https from: loclahost, internalto: External

2/Allow Protocols :

protocols : all trafficsfrom: localhost, internalto: localhost, internal

3/Defaul Rule : Block all.

The problem is : i want to block upload file from internal to external so i've made HTTP filter in Allow Internet Access like this : Config HTTP --> Signature : Search in: Request Header 

 Http header: Content-Type:

 Signature: mutipart/form-data

Methods : Block method POST

Unfortunately, it's not work and i dont know why. If i create a rule block web, it's work. Plesase help me. Thanks !