intercept form submit with jQuery

I want to intercept a submit via jQuery and first check if a file is present on the server. If it's present continue with the request, if not display a message and don't send the request. This is what I have:

$("#methodForm").submit(function(e){

    checkIndex('upload/segments.gen').done(function() {
        return true;
    }).fail(function () {
        e.preventDefault();
        alert("No index present!");
        return false;
    });
});

this is the checkIndex():

function checkIndex(file){
    return $.ajax({
        url : file,
        type:'HEAD'
    });
}

What happens is this: The file is present on the server, but the checkIndex returns with fail. First I see the alert popup and then it continues and sends the post request to the server.

I use the checkIndex() for other purposes as well where it works like expected so I'm pretty sure the error is somewhere in the submit routine. But I can't find out what's wrong with it.