The certain answer to your question can easily be found at www.irs.gov. Within less than a minute I found in Pub. 17 this rule: "Divorced persons. If you are divorced under a final decree by the last day of the year, you are considered unmarried for the whole year." Filing status is determined as of the last day of the year so, no, you can't file your 2012 tax return jointly.
For the tax year in which the divorce became final, it is wise to use a tax professional even if you haven't needed that help in the past.
Best wishes for an outcome you can accept, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
No, your status filing status is determined based on what your status is on December 31, 2012. If you were divorced in September 2012, then you cannot filed jointly. You can either file head of household (depending on who resides with you) or single.