To help with performance testing, the TDML runner needs to be able to specify a documentPart of type="file", for example, something like this:

<tdml:document>

   <tdml:documentPart type="file">path/to/file/relative/to/resources/file.ext</tdml:documentPart>

</tdml:document>

Note that it is document part, so you should be able to combine other documentParts (i.e. more file parts, or byte/text parts)