Unable to redirect to Custom Error page

Hi,
I have created a custom error page to redirect whenever any error occurs. I made the following changes to the web.config file
        <customErrors mode="On">            
            <error statusCode="403" redirect="~/Error.aspx" />
            <error statusCode="404" redirect="~/Error.aspx" />
        </customErrors>  
Suppose my url is in the following way: http://MyTestSite/Posts/Post1.aspx
If I remove the .aspx at the end manually then it is not displaying my customm error page instead it is dispalying default 404 error page : Page cannot be found.
Also, if I give http://MyTestSite/Posts the same 404 error message is displayi ...