The manifest file allows you to access your panorama offline. The online book, Dive Into HTML5, describes this quite well:
One of the most common problems is the wrong mime type for the manifest file. You need to make sure that your webserver is serving the file with the type text/cache-manifest.
If you are using Apache as your web server you can add the following line to your .htaccess file to make sure that the correct mime type is used
AddType text/cache-manifest .manifest
To test the mime type you can use for example the command line tool curl with following line:
curl -I http://example.com/your.manifest