title: How can I add an extension to the "Open With..." dialog
body: <p>My goal is to add a markdown rendering extension to the new "Open With..." dialog in VS Code so that it will open the file using the extension by default. This option is displayed when right-clicking a file in the Explorer panel. Since this is new in VS Code I haven't been able to find any documentation on how extensions can make use of the dialog. Although it is possible that I've just missed it.</p>
  
  <p>These are my current options when used on a markdown file:</p>
  
  <p><img src="https://i.stack.imgur.com/k8oPH.png" alt="Option 1: Text Editor. Option 2 Markdown Preview (Experimental)"></p>
  
  <p>So my question is: where would I start if I wanted to add an extension to this list? I can't imagine it will be that much of a code change. And seeing as this can already be done with the built-in Markdown Preview I assume there must be some way I can accomplish this.</p>