Yes, I thought about it, and maybe it is better to keep file_key. Let me explain:
ImagesPipeline inherits from FilesPipeline and FilesPipeline internals requires file_key.

If someone, by accident removes "def file_key" from ImagesPipeline then image_key will never be used and results will be wrong.

So, here, by using file_key, you test also a pointer from file_key to image_key in ImagesPipeline.