Thanks for the help.. I found the issue.. When I set aspNetCompatibilityEnabled="true" in web.config file of WCF service, I got the HttpPostedFile.

should try this

<configuration>
  <system.serviceModel>
    <serviceHostingEnvironment aspNetCompatibilityEnabled="true"/>
  </system.serviceModel>
</configuration>

or

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Required)]

Thanks..