Forms Data Controls :: File Upload Not Working For Desktop Files?

Iam using a file upload control for uploading files in my asp.net application. iam using the following code to impersonate the users who do not have permission for the files to upload. The code works fine for all the files, but it is not working for the files which are in desktop.

Code in .cs file:

System.Security.Principal.WindowsImpersonationContext impersonationContext;
impersonationContext =
((System.Security.Principal.WindowsIdentity)User.Identity).Impersonate();

//Insert your code that runs under the security context of the authenticating user here.

impersonationContext.Undo();

In web.config iam using the following:

<identity impersonate="true" />