$create() factory method instantiates a
FileSelector given the 3 fields
base_dir: The directory in which to select files. If the path exists but doesn't point to a directory, this should be an error.
allow_not_found: The behavior if
base_dirdoesn't exist in the filesystem. If
FALSE, an error is returned. If
TRUE, an empty selection is returned
recursive: Whether to recurse into subdirectories.