I had a devil of a time trying to get Xcode to recognise my standard Core Data UIManagedDocuments, given they are bundles, not flat files. Turns out one of the keys was the your UTI must inherit from com.apple.package. I had rather more sensibly (IMHO) inherited from one of the built-in database UTIs. Silly me for thinking UTIs should describe the content meaningfully.
Kudos to “cwalcott” of Draconis Software for the iOS Dev Notes blog which finally clued me in to this arbitrary, undocumented requirement.