Ok, answering my own question, in case anyone else comes across this…
the ContentData object does not have a __toString method, so in order to access the mimetype/size/etc, you have to access the properties via $value->mimetype, $value->size, etc.
i just modified that line in the test script to be something like
<td><?php
if(is_string($value))
echo $value;
else
echo $value->mimetype;
?></td>