Plone 4.3 migration fix

After migrating to Plone 4.3, plone.namedfile did not properly handle scaled image data, when images used a lot of data.

Data is provided as zope.app.file.file.FileChunk object by the backend and this is not neither string-like or file-like object. 

The result was that image scales are throwing exceptions which were masked as LocationError.

This patch fixes the behavior by taking FileChunk special case to account when creating image scales.