public ImmutableMap<String, ?> defaultValues(Map<String, ?> userDefaults) {
    return ImmutableMap.of();
  }
Returns a map containing the default attribute values for this provider. The keys of the map
are attribute identifier strings (in "view:attribute" form) and the value for each is the
default value that should be set for that attribute when creating a new file.

<p>The given map should be in the same format and contains user-provided default values. If the
user provided any default values for attributes handled by this provider, those values should
be checked to ensure they are of the correct type. Additionally, if any changes to a
user-provided attribute are necessary (for example, creating an immutable defensive copy), that
should be done. The resulting values should be included in the result map along with default
values for any attributes the user did not provide a value for.
jimfs/src/main/java/com/google/common/jimfs/AttributeProvider.java