Code review comment for lp://staging/~michihenning/thumbnailer/vs-thumb-refactor

I guess the filename change doesn't really matter that much, since we don't use that mode in production.

It still has a bit of a bad smell about it. I'd be happier if we always appended ".tiff" (in which case we never write to the requested file), or never append the extension (in which case we always write to the requested file).