The attack took place on December 7th 2008 when a man was violently assaulted by several other men at 4am on a Noctilien bus in the north of the city. The unprovoked attack left him unable to work for several days.
CCTV footage on the bus meant the culprits could be identified. They were later convicted of the attack.
The 30-year-old policeman made a copy of the footage on a USB stick and then uploaded it onto his Facebook page, with his own comments about how policing has changed. His security settings meant that anyone visiting his page could see the film, not just his 109 Facebook friends.
From Facebook, the footage found its way to other social media sites including YouTube and Dailymotion. It also began appearing on some sites supporting the far-right Front National party.
The president of the Front National, Marine Le Pen, leapt to the defence of the imprisoned policeman on Wednesday, offering him her "complete support."
"His conviction is aimed at intimidating those who say and show the truth about the climate of growing violence and laxness that prevails in France," she said in a statement.
The policeman was also forced to pay €5,000 to the victim of the attack, who said the posting of the film had "abruptly rekindled the trauma" and had been a "serious blow" to his recovery.