The media type of the file content, e.g. 'text/plain' or 'application/json',
see https://www.iana.org/assignments/media-types/media-types.xhtml.
For text files, the charset should be specified in the 'charset' attribute,
e.g. 'text/plain; charset=utf-8'.
Typically 'stdout' or 'stderr' but may also be used for attaching other log output