Starting with the 2009-10 academic year, the French Ministry of Education has eliminated the IUFM assistantships, leaving only the 7 month (secondary and primary) and 9 month (primary) positions.
The 9 month positions are continuously decreased every year as well, so eventually the teaching assistantship will only offer 7 month positions at either the secondary or primary level.
If accepted applicants put IUFM as a preference, obviously it will be ignored. The Ministry of Education just recently eliminated the positions, which is why the IUFM option was still on the application.