prepareFileAttrs = function(o) {
    o = o || {};
    if (o.base64 === true && (o.binary === null || o.binary === undefined)) {
        o.binary = true;
    }
    o = extend(o, defaults);
    o.date = o.date || new Date();
    if (o.compression !== null) o.compression = o.compression.toUpperCase();

    return o;
}
Transforms the (incomplete) options from the user into the complete
set of options to create a file.
@private
@param {Object} o the options from the user.
@return {Object} the complete set of options.