I lost all files?

Hi guys,

I run Mattermost on Ubuntu 16.04.2 LTS

I wanted to upgrade from 3.4.0 to 3.6.2

I backup up my config.json... in fact I copied the full "old" mattermost folder.

I downloaded and extracted the file to /opt/mattermost/

Then copied my config.json file

I started it, and everything was there : the teams, the channels, the users... ALL except my files.
I'm referring to all the PDFs and other documents that were posted on Mattermost.

Then I noticed I didn't copy the data file located in /opt/mattermost/bin/data

So I copied it... rebooted... but still it doesn't work.

Any clue?