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

Question

Python 2

From the Python 2 documentation, while serializing (i.e. writing to a pickle file), use:

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

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

Python 3

The example in the Python 3 documentation now uses .pickle as the file extension for serialization:

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

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?