fix handling of format chains in Chooser

There are two issues in the `Chooser` class since b47351b6b59a60519fec0fe714a7008796165b9e which prevent a proper handling of format chains:

* `addFile()` passes a `FormatInterface` instance to the `File` constructor if the `$formatChain` argument is not `null`
* `getPreferredFilesOrdered()` calls an undefined method `getFormat()` on the passed `FileInterface` instance when unsupported formats are excluded