How do Live Photos work technically? How do Live Photos work technically?

For example: what is the file extension for the captured photos? Is it a sidecar file? How many additional photos are captured before and after the main photo?

I would like some authoritative technical documentation as a reference.
According to 9to5mac:


  Details of the file format were tweeted by Daniel Matte after
  speaking with an iPhone product manager, and are consistent with a
  brief description in Apple’s iOS Developer Library.


The docs don't go into details, but it seems like it's just a set of files:


  The data of a Live Photo is exported as a set of files in a
  PHAssetResource object. The set of files must be preserved as a unit
  when you upload them to a server.


It's a bit disappointing, because they doesn't sound very interoperable or convenient.