replace an instance uploading an xml file

Hello,

I'm trying to use the upload control to select an xml file that must replace
an instance.

The initial instance data is loaded from "inital.xml". I want to give the
user the option to load his own personal data instead of filling the form. I
suppose that xforms is prepared for this, but how?

If I try this:

<xforms:upload ref="instance('the_instance')">
     <xforms:label>File to upload</xforms:label>
</xforms:upload>

Mozilla says:
Error: XForms Error (40): upload element not bound to valid datatype.  Must
be bound to one of these datatypes or a type derived from one of these
datatypes: xsd:anyURI xsd:base64Binary xsd:hexBinary.


If I try this:
<xforms:upload bind="the_intance">
     <xforms:label>File to upload</xforms:label>
</xforms:upload>

Mozilla says:
Error: XForms Error (8): id (the_instance) does not refer to a bind element