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
<!DOCTYPE HTMLin 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 or text/xml. However, the better idea (if you use Cached Copies) is to activate storing Content-Type using a proper Section command in indexer.conf.