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

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

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