Why does Fuchsia OS uses .elf for kernel image?

After building zircon, I get

lz@vm:~/fuchsia/out/default/kernel_arm64$ ls -latotal 73892drwx------  4 lz lz     4096 jul 26 02:36 .drwxrwxr-x 36 lz lz    20480 jul 26 02:39 ..drwx------  4 lz lz     4096 jul 26 01:23 gen-rwxrwxr-x  1 lz lz  3044192 jul 26 02:36 image-rw-rw-r--  1 lz lz        0 jul 26 02:36 image.build-id.stamp-rw-rw-r--  1 lz lz     3442 jul 26 02:36 image.map-rwxrwxr-x  1 lz lz  2178064 jul 26 02:36 kernel.zbidrwx------  8 lz lz     4096 jul 26 02:32 obj-rw-rw-r--  1 lz lz    40021 jul 26 01:23 toolchain.ninja-rwxrwxr-x  3 lz lz 66370040 jul 26 02:36 zircon.elf-rw-rw-r--  1 lz lz       16 jul 26 02:36 zircon.elf.build-id.stamp-rw-rw-r--  2 lz lz    36414 jul 26 01:03 zircon.elf-gdb.py-rw-rw-r--  1 lz lz  3944049 jul 26 02:36 zircon.elf.map

As you see, it creates a .elf zircon kernel (image is also a .elf but I don't know what is the difference from zircon.elf).

I've seen ARM kernels in COFF format but not in ELF. Why does Fuchsia's kernel Zircon produces an elf file?