WP_Image_Editor::get_output_format( string $filename = null, string $mime_type = null )

	protected function get_output_format( $filename = null, $mime_type = null ) {
		$new_ext = null;

		// By default, assume specified type takes priority
		if ( $mime_type ) {
			$new_ext = $this->get_extension( $mime_type );
		}

		if ( $filename ) {
			$file_ext = strtolower( pathinfo( $filename, PATHINFO_EXTENSION ) );
			$file_mime = $this->get_mime_type( $file_ext );
		}
		else {
			// If no file specified, grab editor's current extension and mime-type.
			$file_ext = strtolower( pathinfo( $this->file, PATHINFO_EXTENSION ) );
			$file_mime = $this->mime_type;
		}

		// Check to see if specified mime-type is the same as type implied by
		// file extension.  If so, prefer extension from file.
		if ( ! $mime_type || ( $file_mime == $mime_type ) ) {
			$mime_type = $file_mime;
			$new_ext = $file_ext;
		}

		// Double-check that the mime-type selected is supported by the editor.
		// If not, choose a default instead.
		if ( ! $this->supports_mime_type( $mime_type ) ) {
			/**
			 * Filters default mime type prior to getting the file extension.
			 *
			 * @see wp_get_mime_types()
			 *
			 * @since WP-3.5.0
			 *
			 * @param string $mime_type Mime type string.
			 */
			$mime_type = apply_filters( 'image_editor_default_mime_type', $this->default_mime_type );
			$new_ext = $this->get_extension( $mime_type );
		}

		if ( $filename ) {
			$dir = pathinfo( $filename, PATHINFO_DIRNAME );
			$ext = pathinfo( $filename, PATHINFO_EXTENSION );

			$filename = trailingslashit( $dir ) . wp_basename( $filename, ".$ext" ) . ".{$new_ext}";
		}

		return array( $filename, $new_ext, $mime_type );
	}