You shouldn't need to specify a custom one. GitHub Pages will automatically serve the file with the appropriate mime type given its file extension. Here [1] is

You shouldn't need to specify a custom one. GitHub Pages will automatically serve the file with the appropriate mime type given its file extension. Here [1] is the list. [1]: https://github.com/jekyll/jekyll/blob/master/lib/jekyll/mime.types https://github.com/jekyll/jekyll/blob/master/lib/jekyll/mime... Edit: As you can see in that link, both .manifest and .appcache file extensions map to text/cache-manifest mime type.