Source

File: wp-admin/includes/image-edit.php

function wp_stream_image( $image, $mime_type, $attachment_id ) {
	if ( $image instanceof WP_Image_Editor ) {

		/**
		 * Filters the WP_Image_Editor instance for the image to be streamed to the browser.
		 *
		 * @since 3.5.0
		 *
		 * @param WP_Image_Editor $image         The image editor instance.
		 * @param int             $attachment_id The attachment post ID.
		 */
		$image = apply_filters( 'image_editor_save_pre', $image, $attachment_id );

		if ( is_wp_error( $image->stream( $mime_type ) ) ) {
			return false;
		}

		return true;
	} else {
		/* translators: 1: $image, 2: WP_Image_Editor */
		_deprecated_argument( __FUNCTION__, '3.5.0', sprintf( __( '%1$s needs to be a %2$s object.' ), '$image', 'WP_Image_Editor' ) );

		/**
		 * Filters the GD image resource to be streamed to the browser.
		 *
		 * @since 2.9.0
		 * @deprecated 3.5.0 Use {@see 'image_editor_save_pre'} instead.
		 *
		 * @param resource $image         Image resource to be streamed.
		 * @param int      $attachment_id The attachment post ID.
		 */
		$image = apply_filters_deprecated( 'image_save_pre', array( $image, $attachment_id ), '3.5.0', 'image_editor_save_pre' );

		switch ( $mime_type ) {
			case 'image/jpeg':
				header( 'Content-Type: image/jpeg' );
				return imagejpeg( $image, null, 90 );
			case 'image/png':
				header( 'Content-Type: image/png' );
				return imagepng( $image );
			case 'image/gif':
				header( 'Content-Type: image/gif' );
				return imagegif( $image );
			default:
				return false;
		}
	}
}