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.

lib/FormatInterface.php

@@ -66,6 +66,13 @@ interface FormatInterface {
 	 */
 	public function getExtraInfos();
 
+	/**
+	 * Return MIME type
+	 *
+	 * @return string The MIME type
+	 */
+	public function getMimeType();
+
 	/**
 	 * Set charset
 	 *