Source

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

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

		try {
			// Temporarily change format for stream.
			$this->image->setImageFormat( strtoupper( $extension ) );

			// Output stream of image content.
			header( "Content-Type: $mime_type" );
			print $this->image->getImageBlob();

			// Reset image to original format.
			$this->image->setImageFormat( $this->get_extension( $this->mime_type ) );
		} catch ( Exception $e ) {
			return new WP_Error( 'image_stream_error', $e->getMessage() );
		}

		return true;
	}