For binary base64-encoded data, setting the charset to utf-8 for the data uri doesn't seem to be relevant. You'd only want this if it contained encoded text.
You'll also confuse people using LESS, by thinking they should add the "charset=utf-8" bit as part of the mime type, which turns off base64-encoding using that function, and thus prevents it from being encoded to a data uri.
For binary base64-encoded data, setting the charset to utf-8 for the data uri doesn't seem to be relevant. You'd only want this if it contained encoded text.
You'll also confuse people using LESS, by thinking they should add the "charset=utf-8" bit as part of the mime type, which turns off base64-encoding using that function, and thus prevents it from being encoded to a data uri.