Both filet and fillet mean a strip of boneless meat. Fillet is the more general term, however, while filet is usually reserved for French cuisine and in the names of French-derived dishes such as filet mignon. Dictionaries list filet as also an American variant of the more general fillet, and American writers are indeed inconsistent on the matter. Some use filet even in contexts unrelated to French cuisine, and some use fillet. The same is true of Canadian writers. Outside North America, fillet is much more heavily favored.
In the U.S. and Canada, filet doesn’t always relate to French cuisine. Instances like these are common:
Divide the vegetables evenly among four plates, and top each with a sea bass filet. [Chicago Tribune]
There was a good filet of trout – from Cedar Creek Farm – but too much else on the plate to detract from its goodness … [Ottawa Citizen (article now offline)]
But fillet is used just as often in similar contexts—for example:
That’s perfect, since a thick 4-ounce piece of fish fillet takes 10 to 15 minutes to cook. [San Francisco Chronicle]
He said sockeye fillet now retails for close to $22 a kilogram. [CBC.ca]