DefaultContentType is useful when
the Content-Type header is not
configured using a Section command in indexer.conf
and thus is not stored in the database.
When generating excerpts from a Cached Copy,
search.cgi tries first to fetch Content-Type
information from the database. If no information is found,
search.cgi tries to detect Content-Type automatically
using some well-know substrings like
in the beginning of the
document, unpacked from the Cached Copy
If automatic guesser fails to detect the document type,
it chooses text/plain by default.
With help of DefaultContentType you can change
the default type, for example, to text/html
However, the better idea (if you use Cached Copies) is to
activate storing Content-Type
using a proper Section command in indexer.conf.