尝试一下:

jQuery(document).on('click', '#submit', function(e){
    e.preventDefault();

    var fd = new FormData();
    var file = jQuery(document).find('input[type="file"]');
    var caption = jQuery(this).find('input[name=img_caption]');
    var individual_file = file[0].files[0];
    fd.append("file", individual_file);
    var individual_capt = caption.val();
    fd.append("caption", individual_capt);  
    fd.append('action', 'fiu_upload_file');  

    jQuery.ajax({
        type: 'POST',
        url: fiuajax.ajaxurl,
        data: fd,
        contentType: false,
        processData: false,
        success: function(response){

            console.log(response);
        }
    });
});

php

function fiu_upload_file(){

    var_dump($_FILES);
    exit();
}

add_action('wp_ajax_fiu_upload_file', 'fiu_upload_file');
add_action('wp_ajax_nopriv_fiu_upload_file', 'fiu_upload_file');