Reads the file comment from a folderitem.
Linux and Windows systems have no concept for file comments so this function is not supported on those platforms. It is perfectly valid to call this function on the unsupported platforms but it will only return a empty string there.
MacOS X has no API to read or write OS X comments, so we send a AppleEvent to the operating system to ask the operating system to do it. Because of this fact then reading and writing file comments is slower on MacOS X.