Base64 Image works in Designer, but not WS

In the designer I was able to insert a new image producing the following definition in my XFD file:

<xfd:attribute-value object="16B8637C" attribute="fo:src" dynamic-value="/document/metadata/logo"/>

<fo:external-graphic src="" content-width="scale-to-fit" content-height="scale-to-fit" scaling="uniform" id="16B8637C"/></fo:block>

Clicking on the PDF preview button produces the correct PDF output with images included.

However, after exporting the XFD to XSL, and sending it over SOAP to the Ecrion WS - I get the PDF but without images.

In both cases the same content XML is used.

Here is a snippet from the generated XSL mentioned above:

<fo:external-graphic src="url()" content-width="scale-to-fit" content-height="scale-to-fit"
                     scaling="uniform" id="16B8637C">
    <xslt:attribute name="xf:compat-id">id1689573<xslt:value-of select="generate-id(.)"/>
    </xslt:attribute>
    <xslt:attribute name="id">16B8637C</xslt:attribute>
    <xsl:attribute name="src">
        <xsl:value-of select="/document/metadata/logo"/>
    </xsl:attribute>
</fo:external-graphic>
 
Replacing the xsl:value-of line with this works:
<xsl:value-of select="concat('url(data:image/jpg;base64,',/document/metadata/logo,')')"/>


How can I get the XF designer to automatically generate the required "url()" wrapper and perhaps also the "data:image/jpg;base64" prefix as well without having to modify the exporting XSL file in order to get images working.

Thank you very much for the help.