Provides inflation to a Path::Class::File object allowing file system
storage of BLOBS.

The storage path is specified with fs_column_path. Each file receives
a unique name, so the storage for all FS columns can share the same path.

Within the path specified by fs_column_path, files are stored in
sub-directories based on the first 2 characters of the unique file names.
Up to 256 sub-directories will be created, as needed. Override
_fs_column_dirs in a derived class to change this behavior.

fs_new_on_update will create a new file name if the file has been updated.