avoid illegal url

we develope web site say www.abc.com

if someone type www.abc.com/someDir/somepage.aspx

then it must redirect to our custom error page bcz of someDir,somepage.aspx is not valid

for this we add line to web config as

<customErrors mode="On" defaultRedirect="~\error\error404.html">
   <error statusCode="403" redirect="~\error\error404.html"/>
   <error statusCode="404" redirect="~\error\error404.html"/>
  </customErrors>

but i want that is anyone type www.abc.com/someDir/somepage.txt

other than aspx it must goes to error page
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