Default.aspx change into Default

I am working on the new project web apps using Visual Studio 2013 and when I click the "default file web app" for view in Browser, I found in the URL address the "default.aspx" change into "default" without aspx.  Why this is happening? Also I have another wep apps like "contact.aspx" only show "contact" and not showing "contact.aspx" ????
Thanks