JSFiddle - React, Tailwind, and code Playground

HTML

<form action="/gallery/create" method="post" enctype="multipart/form-data">
   <label>Type:*</label>
    <label for="type-1">
     <input type="radio" checked="checked" value="1" id="type-1" name="type">Image
    </label>
   <label for="type-2">
   <input type="radio" value="2" id="type-2" name="type">Video
    </label>  <br />
   <label class="itemdetailfloatL required" for="file">File:*</label>
   <input type="hidden" id="MAX_FILE_SIZE" value="8388608" name="MAX_FILE_SIZE">
   <input type="file" tabindex="5" class="text-size text" id="file" name="file">
    <br/>
 <input type="submit" value="Create" id="submit" name="submit">
</form>

JavaScript

function getExtension(filename) {
    var parts = filename.split('.');
    return parts[parts.length - 1];
}

function isImage(filename) {
    var ext = getExtension(filename);
    switch (ext.toLowerCase()) {
    case 'jpg':
    case 'gif':
    case 'bmp':
    case 'png':
        //etc
        return true;
    }
    return false;
}

function isVideo(filename) {
    var ext = getExtension(filename);
    switch (ext.toLowerCase()) {
    case 'm4v':
    case 'avi':
    case 'mpg':
    case 'mp4':
        // etc
        return true;
    }
    return false;
}

$(function() {
    $('form').submit(function() {
        function failValidation(msg) {
            alert(msg); // just an alert for now but you can spice this up later
            return false;
        }

        var file = $('#file');
        var imageChosen = $('#type-1').is(':checked');
        if (imageChosen && !isImage(file.val())) {
            return failValidation('Please select a valid image');
        }
        else if (!imageChosen && !isVideo(file.val())) {
            return failValidation('Please select a valid video file.');
        }

        // success at this point
        // indicate success with alert for now
        alert('Valid file! Here is where you would return true to allow the form to submit normally.');
        return false; // prevent form submitting anyway - remove this in your environment
    });

});