In most contexts, / is used to separate directory and file names in filenames, and any use of other conventions (for example, that the user might type on the command line) is converted to that form. The only exceptions might be a few cases in which the server provides a magic cookie which the client then repeats verbatim. Directory names may contain a drive letter in DOS format at the beginnin, as in e:/foo.
Characters outside the invariant ISO 646 character set should be avoided in filenames. This restriction may need to be relaxed to allow for characters such as [ and ] ; this has not been carefully considered (and currently implementations probably use whatever character sets that the operating systems they are running on allow, and/or that users specify). Of course the most portable practice is to restrict oneself further, to the POSIX portable filename character set as specified in POSIX.1.