Fix attachment_filename in flask and add python-dotenv

[thomasaull]

This fixes two small issues:

• attachment_filename was replaced with download_name in flask (see: https://github.com/pallets/flask/pull/4667)
• On startup the server logged: Tip: There are .env or .flaskenv files present. Do "pip install python-dotenv" to use them so I added it to the requirements.txt