reading xml file using ora:readFile('filename')

I read one of the articles 'XML Manipulation'. It talks about using ora:readFile('filename') to read the contents of a file in SOA/BPEL.. If I use the hard-coded filename, like ora:readfile('file:///d:myfile.txt') it will work. But my filename is dynamic so I tried to do the following and will not work
<copy>
<from expression="ora:readFile(concat('file:///', bpws:getVariableData('Variable_File_Name')))"/>
<to variable="Variable_File_Contents"/>
</copy>
can someonw tell lme how I can pass an argument into readFile() as the filename?

Thanks!

SA