formats: Add getMimeType() function (#1299)

Allows getting the expected MIME type of the format's output. A
corresponding MIME_TYPE constant is also defined in FormatAbstract for
the format implementations to overwrite.

formats/AtomFormat.php

@@ -7,6 +7,8 @@
  * https://validator.w3.org/feed/
  */
 class AtomFormat extends FormatAbstract{
+	const MIME_TYPE = 'application/atom+xml';
+
 	const LIMIT_TITLE = 140;
 
 	public function stringify(){
@@ -147,7 +149,7 @@ EOD;
 
 	public function display(){
 		$this
-			->setContentType('application/atom+xml; charset=' . $this->getCharset())
+			->setContentType(self::MIME_TYPE . '; charset=' . $this->getCharset())
 			->callContentType();
 
 		return parent::display();