title: Possible to add MIME type to web.config without possibly breaking the site?
body: <p>I had a web.config in one of the websites on my IIS that was adding a support for .7z file extension. When I later added a global .7z support at the server level, this site was broken - IIS Manager is complaining that it "cannot add duplicate collection entry of type 'mimeMap'..." and all web requests to i.g. CSS files ended with an HTTP 500 error.</p>
  
  <p>I was using this in the site's web.config:</p>
  
  <pre><code>&lt;system.webServer&gt;
      &lt;staticContent&gt;
          &lt;mimeMap fileExtension=".7z" mimeType="application/x-7z-compressed" /&gt;
      &lt;/staticContent&gt;
  &lt;/system.webServer&gt;
  </code></pre>
  
  <p>Is there maybe some other syntax that would add 7z to the list only if it wasn't defined yet?</p>