Add getMimeType to Output & other small adjustments

src/output.ts

@@ -344,6 +344,15 @@ export class Output<
 		})();
 	}
 
+	/**
+	 * Resolves with the full MIME type of the output file, including track codecs.
+	 *
+	 * The returned promise will resolve only once the precise codec strings of all tracks are known.
+	 */
+	getMimeType() {
+		return this._muxer.getMimeType();
+	}
+
 	/**
 	 * Cancels the creation of the output file, releasing internal resources like encoders and preventing further
 	 * samples from being added.