title: Java ScriptEngineManager returns null when using custom ScriptEngine
body: <p>I previously asked <a href="https://softwareengineering.stackexchange.com/questions/277695/how-to-create-methods-at-runtime-for-scripting-purposes">this question</a> on StackExchange. Because it was put on hold, I'm following the comments and asking about how to fix this instead. To avoid repeating myself and copy pasting an explanation, please read the previous question I asked for the full details.</p>
  
  <p>I'm trying to write my own custom ScriptEngine for Java. I tried extending the <code>AbstractScriptEngine</code> and implementing <code>ScriptEngineFactory</code> and filling out all the bare minimum required methods just to test if I'd be able to at least start working with scripts. (See the code <a href="https://github.com/Gamefreak0/SandboxScript/tree/master/src/main/java/me/gamefreak0/ss" rel="nofollow noreferrer">HERE</a>) When I ran the following code...</p>
  
  <pre><code>public static void main(String[] args) {
      ScriptEngineManager manager = new ScriptEngineManager();
      ScriptEngine engine = manager.getEngineByExtension("sbs"); // &lt;- THIS LINE HERE
  
      System.out.println(engine);
  
      try {
          System.out.println(engine.eval("STRING!"));
      } catch (ScriptException e) {
          e.printStackTrace();
      }
  </code></pre>
  
  <p>The <code>manager.getEngineByExtension("sbs");</code> method returns null. I have also tried using <code>getEngineByMimeType("text/SandboxScript");</code> as well as <code>getEngineByName("SandboxScript");</code> but none of these have worked. </p>
  
  <p>Is there something I set up incorrectly?</p>