title: Find files with a given extension in a directory tree
body: <p>I have a folder in which I have some files and sub-folders. I need to return the names of the files or relative address (if the file is in sub-folder) that have a given extension, for example <code>.html</code>.</p>
  
  <p>For example this is structure of given folder:</p>
  
  <ul>
  <li>/text/something.ncx</li>
  <li>/text/123.html</li>
  <li>content.opf</li>
  <li>toc.ncx</li>
  <li>Flow_0.html</li>
  <li>Flow_1.html</li>
  <li>Flow_2.html</li>
  </ul>
  
  <p>So the code should return this (in an array ideally):</p>
  
  <ul>
  <li>text/123.html</li>
  <li>Flow_0.html</li>
  <li>Flow_1.html</li>
  <li>Flow_2.html</li>
  </ul>
  
  <p>It's just pulling out names with <code>.html</code> extension, but I really don't how to solve this.</p>