次のプログラムを改造して、添付ファイル付きのメールを送るようにしました。
Gmail の oauth2 でメールの送信 (python3)

まず、credentials.json　を取得します。

Gmail->API->Guides->Python Quickstart

get_credentials.py

#
#   get_credentials.py
#
#                   Jan/03/2019
# ------------------------------------------------------------------
import httplib2
import os
import sys

import oauth2client
import argparse
from oauth2client import file, client, tools
# ------------------------------------------------------------------
SCOPES = "https://www.googleapis.com/auth/gmail.send"
CLIENT_SECRET_FILE = "credentials.json"
APPLICATION_NAME = "MyGmailSender"

# ------------------------------------------------------------------
# [6-4]:
def get_credentials_proc(flags):
    script_dir =os.path.abspath(os.path.dirname(__file__)) 
    credential_dir = os.path.join(script_dir, ".credentials")

    if not os.path.exists(credential_dir):
        os.makedirs(credential_dir)
    credential_path = os.path.join(credential_dir,"my-gmail-sender.json")

    store = oauth2client.file.Storage(credential_path)
    credentials = store.get()
    if not credentials or credentials.invalid:
        flow = oauth2client.client.flow_from_clientsecrets(CLIENT_SECRET_FILE, SCOPES)
        flow.user_agent = APPLICATION_NAME
        credentials = oauth2client.tools.run_flow(flow,store,flags)
        print("Storing credentials to " + credential_path)
#
    return credentials
#
# ------------------------------------------------------------------

gmail_attachement.py

#
#   gmail_attachement.py
#
#                   Jan/03/2019
# ------------------------------------------------------------------
import httplib2
import os
import sys

import apiclient
from get_credentials import get_credentials_proc
# ------------------------------------------------------------------
import base64
from email.mime.multipart import MIMEMultipart
from email.utils import formatdate
from email.mime.base import MIMEBase
import traceback
from email.message import EmailMessage
import imghdr

# ------------------------------------------------------------------
# [6-8-4]:
def attach_image_proc(message,img_data,filename):
    try:
        message.add_attachment(img_data,maintype='image',
            subtype=imghdr.what(None,img_data),filename=filename)
    except Exception as ee:
        sys.stderr.write("*** error *** in message.add_attachment ***\n")
        sys.stderr.write(str(ee) + "\n")
#
    return message
# ------------------------------------------------------------------
# [6-8-6]:
def attach_application_proc(message,data_attach,filename,subtype):
    try:
        message.add_attachment(data_attach,maintype='application',
            subtype=subtype,filename=filename)
    except Exception as ee:
        sys.stderr.write("*** error *** in message.add_attachment ***\n")
        sys.stderr.write(str(ee) + "\n")
#
    return message
# ------------------------------------------------------------------
# [6-8]:
def create_message(mail_from,mail_to,subject,str_message,file_attach):
    message = EmailMessage()
    message.set_content(str_message)
#
    message["From"] = mail_from
    message["To"] = mail_to
    message["Subject"] = subject
    message["Date"] = formatdate(localtime=True)
#
    fp = open(file_attach,'rb')
    img_data = fp.read()
    fp.close()
#
    for suffix in [".pdf",".csv",".json",".xlsx",".zip"]:
        if (file_attach.endswith(suffix)):
            message = attach_application_proc \
                (message,img_data,file_attach,suffix)
#
    for suffix in [".jpg",".png"]:
        if (file_attach.endswith(suffix)):
            message = attach_image_proc \
                (message,img_data,file_attach)
#
    byte_msg = message.as_string().encode(encoding="UTF-8")
    byte_msg_b64encoded = base64.urlsafe_b64encode(byte_msg)
    str_msg_b64encoded = byte_msg_b64encoded.decode(encoding="UTF-8")

    return {"raw": str_msg_b64encoded}
#
# ------------------------------------------------------------------
# [6]:
def gmail_attachement_proc(mail_from,mail_to,subject,str_message,file_attach,flags):
    credentials = get_credentials_proc(flags)
    http = credentials.authorize(httplib2.Http())
    service = apiclient.discovery.build("gmail", "v1", http=http)
#
    try:
        result = service.users().messages().send(
            userId=mail_from,
            body=create_message(mail_from,mail_to,subject,str_message,file_attach)
        ).execute()

        print("Message Id: {}".format(result["id"]))

    except apiclient.errors.HttpError:
        print("------start trace------")
        traceback.print_exc()
        print("------end trace------")
#
# ------------------------------------------------------------------

mail_from　は credentials.json を作成したユーザーにして下さい。
mail_to は送信先です。
file_attach は添付ファイルの名前です。

test_gmail.py

#! /usr/bin/python
#
#   test_gmail.py
#
#                   Jan/03/2019
# ------------------------------------------------------------------
import sys

import argparse
import oauth2client

from gmail_attachement import gmail_attachement_proc 

flags = argparse.ArgumentParser(
    parents=[oauth2client.tools.argparser]
).parse_args()

# ------------------------------------------------------------------
sys.stderr.write("*** 開始 ***\n")
#
mail_from = "example01@gmail.com"
mail_to = "user01@example.or.jp"
#
subject = "Gmail Api Test Jan/03/2019 PM 20:45"
str_message = ""
str_message += "こんにちは。\n"
str_message += "Jan/03/2019 PM 20:45\n"
#
file_attach = "federer.png"
file_attach = "in01.csv"
file_attach = "example01.pdf"
file_attach = "federer.jpg"
#
sys.stderr.write("file_attach = " + file_attach + "\n")
#
gmail_attachement_proc(mail_from,mail_to,subject,str_message,file_attach,flags)
#
sys.stderr.write("*** 終了 ***\n")
# ------------------------------------------------------------------

実行方法

./test_gmail.py

リモートのサーバーのコンソールで最初に実行する時は

./test_gmail.py --noauth_local_webserver

次のバージョンで確認しました。

$ python --version
Python 3.8.2