embed Base64 content into xml element

I have some content in base64 format in a text file. How can I embed
the base64 content into xml file.

For example :

base64 text file:
------------------------
RGVwbG95bWVudFx0ZW1wXE1vZGlmaW

Required output in XML File:
-----------------------------------------

<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
            <env:Body>
                <dp:request
xmlns:dp="http://www.datapower.com/schemas/management">
                    <dp:file> CONTENT FROM BASE64 TEXT FILE </dp:file>
                </dp:request>
            </env:Body>
</env:Envelope>

How can I write XSLT?

Thanks.