Returns the MCRFile or MCRDirectory that is represented by the given
FileObject, which is a direct child of the directory FileObject this
MCRDirectory is stored in.
If the MCRContentStore of the MCRFile given provides an
MCRAudioVideoExtender implementation, this method creates and initializes
the MCRAudioVideoExtender instance for the MCRFile.
Builds a fingerprint of all file's contents of this directory, by collecting the MD5 checksums of all direct or indirect child files of this directory,
sorting them to ascending order and writing this list line by line as UTF-8 encoded bytes.