[swfmill] Bitmap Encoding on <data> element.

[Fred]

I've got a swf2xml which contains something like...

...
   <DefineBitsLossless2 objectID="1" format="5" width="243" height="256">
        <data>
          <data>eNrtnQl8TOf6x3+WCKFS+1ZFaU
...
        vn8vWyJshf8B9JqkqA==</data></data>
      </DefineBitsLossless2>
...

What is the encoding type of the inner data element?