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No, it's to copy file like objects that aren't necessarily true file system objects. The only requirement is that they support a minimal file-like object interface. There is no suggestion or requirement that the object supports file descriptors (which would make it a real os file object).
I think changing it to use os.sendfile if possible would be too risky in terms of causing unexpected breakage (objects that look like they have a file descriptor but really don't), especially since handling file system files is explicitly not the purpose of copyfileobj.