While version have changed over time all the way up to the current v14, the FileMaker file format has also changed a couple of times.
From fp4, fp5, a major change to multi-table with relationship graph in fp7 and most recently to fmp12 (the current format). Format fp5 will work with v5, 5.5 and 6. Format fp7 will work with v7, 8, 8.5, 9, 10 and 11. Format fmp12 will work with v12, 13 and 14.