exporter: How to get :file property on a src block in an exporter

[Yasushi SHOJI]

Hi,

I'm writing an exporter and I'd like to get :file property on a src block.
Let's say I have the following src block in an org file.

#+BEGIN_SRC ditaa :file images/hello-world.png :exports code
+--------------+
| Hello World! |
+--------------+
#+END_SRC"

And, say, I'm writing a function for my exporter:

(defun org-myexporter-src-block (src-block contents info)
    (let ((file (xxxxxxx yyy)))))

How can I get "images/hello-world.png" in org-myexporter-src-block?

Thanks,
-- 
            yashi