application/x-apple-diskimage

The .dmg download that installs most Mac software: a mountable disk image in Apple's UDIF format, optionally compressed block-by-block with zlib, bzip2, ADC, or LZFSE. Apple has never published a specification or registered the type — everything public about UDIF comes from reverse engineering — which is a remarkable state of affairs for the default software-distribution container of a major platform. The identification quirk is that the koly metadata block sits in the last 512 bytes of the file, not the first, so no leading magic-byte signature exists; sniffers must read the tail.

.dmg is modern UDIF; the ancient .smi and .img NDIF images from classic Mac OS are a different format.

Defined by Apple Disk Image — Wikipedia. Registry facts from the IANA media-types registry via mime-db.