Source

File: wp-includes/class-wp-image-editor-gd.php

public function stream( $mime_type = null ) {
		list( $filename, $extension, $mime_type ) = $this->get_output_format( null, $mime_type );

		switch ( $mime_type ) {
			case 'image/png':
				header( 'Content-Type: image/png' );
				return imagepng( $this->image );
			case 'image/gif':
				header( 'Content-Type: image/gif' );
				return imagegif( $this->image );
			default:
				header( 'Content-Type: image/jpeg' );
				return imagejpeg( $this->image, null, $this->get_quality() );
		}
	}