Whats wrong with this XSD?

Hello!

Our customer has provide us with this xsd:
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
    <xs:element name="Document">
        <xs:complexType>
            <xs:attribute name="Id" type="xs:string" use="required"/>
            <xs:attribute name="Name" use="required">
                <xs:simpleType>
                    <xs:restriction base="xs:string"/>
                </xs:simpleType>
            </xs:attribute>
            <xs:attribute name="Category" use="required">
                <xs:simpleType>
                    <xs:restriction base="xs:string"/>
                </xs:simpleType>
            </xs:attribute>
        </xs:complexType>
    </xs:element>
    <xs:element name="DocumentList">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="Document" maxOccurs="unbounded"/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:element name="Folder">
        <xs:complexType>
            <xs:sequence>
                <xs:element ref="DocumentList"/>
            </xs:sequence>
            <xs:attribute name="Id" type="xs:string" use="required">   
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:pattern value="[0-9]{4}-[0-9]{4}"/>
                    </xs:restriction>
                </xs:simpleType>
            </xs:attribute>
        </xs:complexType>
    </xs:element>
</xs:schema>

But during my validation in my Java code with this xsl and in XML Spy I got the message that this XSL is not valid, the message looks like this in XML Spy:

This schema doesn'tmappear to be valid by itself (as part of another schema, it might still be OK):
Schema error - undefind schema component 'xs:simpleType' encountered - only one type or simpleType may be used

It also points/marks the rows:
                <xs:simpleType>
                    <xs:restriction base="xs:string">
                        <xs:pattern value="[0-9]{4}-[0-9]{4}"/>
                    </xs:restriction>
                </xs:simpleType>

So I guess ther must be a problem here.

Do you see the problem?

Best regards
Fredrik