I've been confused for a while on why some files return a HTML version of an error, and others a textual version.
For instance: http://commons.wikimedia.org/wiki/File:Harplute-mechanicaldrawing-large-sidneyrobertsoncowell.png returns an HTML error and http://commons.wikimedia.org/w/index.php?title=File:England_Administrative_1931.tif&page=1 an image url. (the image contains the error report as well, but that is not the point really).
I cannot determine what the difference is between the two render methods. Clearly in one case we get the toHtml of ThumbnailImage class and in the other of MediaTransformError class, but