Steps to reproduce

1. Generate a pickle file "test" (I didn't test other common files yet)

2. Send this file to user

bot.send_document(chat_id=user_chat_id, document=open('./test', 'rb'), filename="test")

or

bot.send_document(chat_id=user_chat_id, document=open('./test', 'rb'))

Expected behaviour

User will receive a file named test

Actual behaviour

User received a file named application.octet-stream

Configuration

Operating System:

Debian (Server, where I first found this issue)

Ubuntu(Local, I test on v10.1.0, everything is fine, so I upgrade to v11.1.0, then I have the same issue as Debian Server)

Version of Python, python-telegram-bot & dependencies:

$ python -m telegram

My Local Ubuntu After Upgrade:
python-telegram-bot 11.1.0
certifi 2018.08.24
future 0.16.0
Python 3.6.6 (default, Sep 12 2018, 18:26:19) [GCC 8.0.1 20180414 (experimental) [trunk revision 259383]]

The pictures shows results of python-telegram-bot v10.1.0 (the first one) and v11.1.0 (the second one) :