You can't do that by using those existing mechanisms since the extraction prefix is always a string constant at runtime and cannot change. It should be possible/easy to do in the next release when using the generic file analysis interface.
For now, what you could do is basically duplicate the code of scripts/base/protocols/http/file-extract.bro except replace usage of "extraction_prefix" with a call to your own function that returns the file prefix that you want (it would probably use something like "strftime("%Y-%m-%d", current_time()" to get the date part of it). Then change your local.bro to use your own version of the script.