Form not submitting to iFrame, opens in new tab

I'm using the following html beginning form code:

<form id="uploadpic" method="post" target="uppic" name="upform" action="/cgi-bin/inpost.cgi" enctype="multipart/form-data">

to try and load a page, however the page opens in a new tab.

I don't think this is part of the problem, but I'll include this anyways. This is the submit button code in the form:

<input name="filename" id="filename" type="file" onchange="submitFormAfterImageCheck();" /> 

And this is the function it calls:

function submitFormAfterImageCheck()
            {
                if(/(\.jpeg|\.jpg|\.JPG|\.gif|\.png|\.tiff)$/.test(document.getElementById("filename").value))
                {
                    nogo = "go";
                    document.getElementById("uploadpic").submit();
                    $("#upload").html("<center>LOADING...</center>");
                    $('#link').hide();
                    $('#update_post').hide();

                }
                else
                {
                    alert("You can only upload an image.");
                }
            }

iFrame code:

<iframe id="uppic" name="uppic"></iframe>

Why isn't this submitting to the iframe?