Adding custom extension (.SEO) to the URL

Add the handler mapping to IIS

 <handlers>
            <add name="*.seo-4.0_32Bit" path="*.seo" verb="*" modules="IsapiModule" scriptProcessor="C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" resourceType="Unspecified" preCondition="classicMode,runtimeVersionv4.0,bitness32" />
 </handlers>

If extension not worked, add following between <system.webServer>

<modules runAllManagedModulesForAllRequests="true" >
<remove name="UrlRoutingModule"/>
</modules>