I got the way to fix it.
Uncomment the following statement in machine.config
<add verb="*" path="*.vsdisco"
, System.Web.Services, Version=1.0.3300.0,
>Recently, I'm learning how to building Web Services. And
>built a Web Services project and tested it successfully.
>And then I created a ASP.NET web application to consume
>web services. But when I used 'Add Web Reference', error
>showed "The document was not recognized as a known
>document type (WSDL, XML Schema, or Discovery document)
>for the following reason: -The content
>type 'application/octet-stream' is not valid for an XML
>documnt." (Please see the attached file).