Major Description
A full-text search on the traversing index falls back to a sort of manual evaluation of results.
This is handled by the FullTextTerm class, and it appears that it passes the constraint text through a cleanup process where it strips most of the characters that are neither Character.isLetterOrDigit(c) not in the list +-:&
I'm not exactly sure where this list comes from, but I see the '/' character is missing which causes a certain type of query to fail.
Example:
//*[jcr:contains(., 'text/plain')]