plugin.xml

<extension
     point="org.eclipse.core.contenttype.contentTypes">
  <content-type
        base-type="org.eclipse.core.runtime.text"
        file-extensions="properties"
        id="tm4.test"
        name="Test file"
        priority="normal">
  </content-type>
</extension>

<extension
     point="org.eclipse.tm4e.registry.grammars">
  <grammar
        path="src/syntaxes/test.tmLanguage.json"
        scopeName="source.test">
  </grammar>
  <scopeNameContentTypeBinding
        contentTypeId="tm4.test"
        scopeName="source.test">
  </scopeNameContentTypeBinding>
</extension>