Adding an attribute to a restricted simple type

Dear XML Schema Users,

Assume the following XML Schema Syntax

<simpleType name="type0">
  <restriction base="string">
    <enumeration value="my value"/>
  </restriction>
</simpleType>

<element name="x">
  <complexType>
    <simpleContent>
      <extension base="type0">
        <attribute name="my_attribute" type="string"/>
      </extension>
    </simpleContent>
  </complexType>
</element>

My Question: is this allowed, i.e., extension and restriction
are (indirectly) combined?

Kind regards,
Maikel.