Preferred (or most common) file extension for a Python pickle

This is a question from a student that I didn't have a good answer for. At times, I've seen .pickle, .pck, .pcl, and .db for files that contain Python pickles but am unsure what is the most common or best practice. I know that the latter three extensions are also used for other things.

The related question is what MIME type is preferred for sending pickles between systems using a REST API?

Python 2

From the Python 2 docs:

output = open('data.pkl', 'wb')

I would choose .pkl as the extension when using Python 2.

Python 3

The example in the Python 3 docs now uses .pickle as the file extension:

with open('data.pickle', 'rb') as f:

The MIME type preferred for sending pickles from martineau's comment below:

application/octet-stream

See What is the HTTP "content-type" to use for a blob of bytes?

From: stackoverflow.com/q/40433474