The Spring config used to define these custom transformers uses the now-deprecated explicitTransformations property. This no longer has any effect as it is ignored by the RuntimeExecutableContentTransformerWorker class, which now always returns true when asked if ANY given mimetype X can be transformed to another mimetype Y.
The result is that any given mimetype is listed in the mimetypes web script (at /service/mimetypes) as being transformable to all other mimetypes configured in the system.
The changes were merged into HEAD in r46719.
The result is that when video-transformation-context.xml.sample and video-thumbnail-context.xml.sample are enabled by removing the .sample extension, the repository believes that ALL mimetypes in the repository can be converted to SWF renditions (and in fact to all other mimetypes).
The SWF rendition is significant because Share is configured to use it as first preference in web previews. Share attempts to load a SWF rendition of any content item in the repository. This fails for many file types such as images and an error is displayed to the user.
More seriously, there are no errors or warnings in the repository logs to indicate that the transformation worker beans were not properly configured.
We should ensure that these sample files work as intended and do not cause the side-effects of all mimetypes being transformable to all others. It is likely that customers and partners will have configured similar beans in their own environments, so even if we fix up the sample files it is likely they will run into problems unless the always-return-true default rule is changed, especially without any errors or warnings.