How can I get an attached eml file from email message content using Python?

Use eml_parser https://pypi.org/project/eml-parser/

import datetime
import json
import eml_parser


def json_serial(obj):
    if isinstance(obj, datetime.datetime):
        serial = obj.isoformat()
        return serial


with open('sample.eml', 'rb') as fhdl:
    raw_email = fhdl.read()

parsed_eml = eml_parser.eml_parser.decode_email_b(raw_email)

print(json.dumps(parsed_eml, default=json_serial))

Just to explain why this is happening (it hit me too), quoting the v. 3.5 library doc. (v2 says the same):

If the message is a multipart and the decode flag is True, then None is returned.

If your attachment is an .EML, it's almost always going to be multi-part, thus the None.

Jin Thakur's workaround is appropriate if you're only expecting .EML multipart attachments (not sure if there is any other use cases); it should have been accepted as an answer.