You only must file your tax return in Spain if you spent more than 6 months and one day in Spain last year. As you have only been resident in Spain since the end of September last year, your tax return must be submitted in France for the whole of last year (in application of article 9.1-a of Law nº 35/2006 of 28 November 2006 on tax returns, in force since 01 January 2007).
