Hi,
this problem is because of MongoDB version 3.4.1 that was released 2016-12-20.
Downgrade to MongoDB 3.4.0, this keeps your existing data, and uploading attachments works with it:
sudo apt-get remove mongodb-org*
sudo apt install -y mongodb-org=3.4.0 mongodb-org-server=3.4.0 mongodb-org-shell=3.4.0 mongodb-org-mongos=3.4.0 mongodb-org-tools=3.4.0
I’ll try to figure out later what has changed in MongoDB 3.4.1.
In Wefork it’s this issue: