title: Using plus sign in custom internet media types (MIME types)
body: <p>I'm building a RESTful web service, and I'm creating some custom media types. It seems like in media types the subtype becomes more specific as you go right to left using '+'. So for example:</p>
  
  <p><code>application/json</code> is more general than <code>application/hal+json</code>, since <a href="http://stateless.co/hal_specification.html" rel="noreferrer">HAL</a> adds more restrictions on top of json.</p>
  
  <p>My question is whether this is standard, or just a convention (I've poked through some of the RFCs on MIME types and I couldn't find anything, but I may not have looked hard enough).</p>
  
  <p>If it is standard, does it generalize so I can add even more specific subtypes like <code>application/vnd.custom.mime+hal+json</code> or is it limited to one "+"?</p>