I've researched but all I can find is that the manifest file should have the correct MIME type & that's text/cache-manifest. I have no idea what a MIME type is.
As stated in WikiPedia:
An Internet media type is a standard identifier used on the Internet to indicate the type of data that a file contains. Common uses include the following:
email clients use them to identify attachment files, web browsers use them to determine how to display or output files that are not in HTML format, search engines use them to classify data files on the web.
A media type is composed of a type, a subtype, and zero or more optional parameters. As an example, an HTML file might be designated text/html; charset=UTF-8. In this example text is the type, html is the subtype, and charset=UTF-8 is an optional parameter indicating the character encoding.
IANA manages the official registry of media types.
The identifiers were originally defined in RFC 2046, and were called MIME types because they referred to the non-ASCII parts of email messages that were composed using the MIME (Multipurpose Internet Mail Extensions) specification. They are also sometimes referred to as Content-types.
Their use has expanded from email sent through SMTP, to other protocols such as HTTP, RTP and SIP.