This sounds like a bug. We shouldn't be making workarounds for issues that exists. We should fix them and build the rest of the code independent of the issues that exists or not in other projects. 

But just for this particular case, why do we need to fix the extension for the new generated cropduster images? If the filer files are working without an extension and are served correctly to the end-user, do we really need to replace/generate extensions if the ones from the original file is invalid?