title: How to add a second extension to a file name?
body: <p>I want to create a data file, but before writing to the final file I want to drop it in a temporary location to avoid user confussion. As an example, I could begin with <code>test.txt</code> and want to have <code>test.txt.tmp</code>. The names could include a path, but the files may not necesarily exist (so this question is purely about string manipulation).</p>
  
  <p>The closest I have been is to use <code>Path.ChangeExtension</code>:</p>
  
  <pre><code>string original = "test.txt";
  string temp = Path.ChangeExtension(original, "tmp");
  </code></pre>
  
  <p>But that returns <code>test.tmp</code> instead. So my question is if there is a built-in method to achieve that "dual-extension" file name? I could always use brain-dead string concatenation, but I'm looking for a more safe and tested method.</p>