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.