If you need to offer a zip file for download you can return one of the following content type.

application/zip
application/x-zip-compressed
application/octet-stream

You can also check for a given MIME type

 

This is how to check for a Content Type of a Zip file.

// ....
if ( ! preg_match( '#application/[(x\-]*zip|octet-stream)#si', $content_type ) ) {
	throw new Exception( "Invalid content type: [$content_type]" );
}
// ....