[Zope] Calling a File from an External Method

[beno]

Hi;
I'm calling a function with a file name from an external method. I need to 
pass a variable to the method, so I have a line like this:

<dtml-var expr="externalMethod('file')">

but the error comes back thus:

IOError: [Errno 2] No such file or directory: 'file'
when said file does indeed exist in the same directory as the DTML Method 
and also the External Method. What have I done wrong?
TIA,
beno