On the page detailing ImageObject:
http://schema.org/ImageObject

The following example is given:
<img src="mexico-beach.jpg" itemprop="contentURL" />

On MediaObject (http://schema.org/MediaObject) we have another example of contentURL use.
<meta itemprop="contentURL" content="http://media.freesound.org/[snip]....preview.mp3" />

The expected type of contentURL is "URL."  All fine and well.  Except that under the description field schema.org says:
"Actual bytes of the media object, for example the image file or video file."

Wouldn't one expect a number here, then?  Or is the description incorrect?

Or am I missing something?  The reference to the "image file or video file" again seems to support URL as the type, but "bytes" does not.