How to make a choice in XSD?

[Sharon]

I'm trying to write an XML schema file (XSD) that will have two kind of
elements:
* xs:base64Binary
* xs:string

But I need that the schema will let only one of this elements to exist in
the XML (or DataSet), meaning; I wand the user to enter or byte[] array data
or a string, but not both.
When the user select one of the two elements type, he can enter as many as
he likes.

I tried the xs:choice that will hold this two elements as follow:
<xs:choice id="PictureType" minOccurs="0" maxOccurs="2">
<xs:element name="Dump">
<xs:complexType>
<xs:sequence>
<!-- Array of Byte objects -->
<xs:element name="PictuteDump" type="xs:hexBinary" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="Path">
<xs:complexType>
<xs:sequence>
<!-- Full path of the picture file -->
<xs:element name="PictutePath" type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:choice>

But still it allow me to eneter both!

Note: The xs:base64Binary is for holding picture data and the string is for
holding the path of a image file saved on disk.

--------
Thanks
Sharon G.