Source

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

protected function _save( $image, $filename = null, $mime_type = null ) {
		list( $filename, $extension, $mime_type ) = $this->get_output_format( $filename, $mime_type );

		if ( ! $filename ) {
			$filename = $this->generate_filename( null, null, $extension );
		}

		if ( 'image/gif' === $mime_type ) {
			if ( ! $this->make_image( $filename, 'imagegif', array( $image, $filename ) ) ) {
				return new WP_Error( 'image_save_error', __( 'Image Editor Save Failed' ) );
			}
		} elseif ( 'image/png' === $mime_type ) {
			// Convert from full colors to index colors, like original PNG.
			if ( function_exists( 'imageistruecolor' ) && ! imageistruecolor( $image ) ) {
				imagetruecolortopalette( $image, false, imagecolorstotal( $image ) );
			}

			if ( ! $this->make_image( $filename, 'imagepng', array( $image, $filename ) ) ) {
				return new WP_Error( 'image_save_error', __( 'Image Editor Save Failed' ) );
			}
		} elseif ( 'image/jpeg' === $mime_type ) {
			if ( ! $this->make_image( $filename, 'imagejpeg', array( $image, $filename, $this->get_quality() ) ) ) {
				return new WP_Error( 'image_save_error', __( 'Image Editor Save Failed' ) );
			}
		} else {
			return new WP_Error( 'image_save_error', __( 'Image Editor Save Failed' ) );
		}

		// Set correct file permissions.
		$stat  = stat( dirname( $filename ) );
		$perms = $stat['mode'] & 0000666; // Same permissions as parent folder, strip off the executable bits.
		chmod( $filename, $perms );

		return array(
			'path'      => $filename,
			/**
			 * Filters the name of the saved image file.
			 *
			 * @since 2.6.0
			 *
			 * @param string $filename Name of the file.
			 */
			'file'      => wp_basename( apply_filters( 'image_make_intermediate_size', $filename ) ),
			'width'     => $this->size['width'],
			'height'    => $this->size['height'],
			'mime-type' => $mime_type,
		);
	}