This allows to filter the indexed documents by the MIME type property of the crawled content. Basically this will allow you to restrict the MIME type of the contents that will be stored in Solr/Elasticsearch index without the need to restrict the crawling/parsing process, so no need to use URLFilter plugin family. Also this address one particular corner case when certain URLs doesn't have any format to filter such as some RSS feeds (http://www.awesomesite.com/feed) and it will end in your index mixed with all your HTML content.
A configuration can file specified on the mimetype.filter.file property in the nutch-site.xml. This file use the same format as the urlfilter-suffix plugin. If no mimetype.filter.file key is found an allow all policy is used instead, so all your crawled documents will be indexed.