Other French evidence gave details of suspected chemical attacks in the towns of Saraqib and Jobar in April which now appeared to have killed about 280 people, the report said.
"We believe the Syrian opposition does not have the capacity to carry out an operation of such magnitude with chemical agents," it concluded.
Speaking after presenting the files to French MPs, Jean-Marc Ayrault, the Prime Minister, said "nobody can deny" the reality of the August attack and that President Francois Hollande was "continuing efforts to forge a coalition as soon as possible" against Syria.
French MPs will debate the Syria crisis tomorrow, although Ayrault said there would be no vote. The President's aides have made it plain that France will not strike alone.
Assad challenged the West to provide "the slightest proof" that he used chemical weapons against his people.
He said any Western military intervention could lead to "regional war" and would harm "the interests of France".
"Whoever accuses must provide proof. We have challenged the United States and France to provide the slightest proof. Obama and Hollande have been incapable [of doing so] even to their own people," Assad told the French newspaper Le Figaro.
"Whoever contributes to financially or militarily bolstering terrorists is an enemy of the Syrian people," he said. "Whoever is against the interests of Syria and its people is an enemy."