I don't think there's anything like that built into SilverStripe, but will be happily corrected if I'm wrong.
One option may be to use a service like Flickr.
But if you wanted to stay in the CMS, a very non-elegant solution might be to create a subpage for each multimedia item. You can use your title field to insert your tags, or create a custom page with a "tags" field. Then you should be able to search for the tags and display them on another page accordingly.
I think this is going to be a more requested addition as more sites with more media start to use SS using the current file hierarchy may get a bit confusing. One way I think you would be able to do this (if you have php knowledge) is to use DataObjectDecorator - http://doc.silverstripe.com/doku.php?id=dataobjectdecorator to decorate the File class with your own CustomFile class and add the fields you require.