A media conversion can be added to your model in the registerModelConversions-function.
It should start with a call to addMediaConversion. From there on you can use any of
the methods available in the API. They are all chainable.
/**
* Set the collection names on which this conversion must be performed.
*
* @paramstring$collectionNames,...
*
* @return$this
*/publicfunctionperformOnCollections($collectionNames)