Warning: Missing boundary in multipart/form-data POST data in Unknown on line 0

Question

I am working on file uploader which upload the image when the input is changed my code for the form in html is

<form method="post" enctype="multipart/form-data">
     <input name="uploaded[]" type="file" id="file_upload"/>
</form>

My JavaScript and Ajax :

        document.getElementById("file_upload").onchange = function() {
            var id = document.getElementById("user_id").innerHTML;
            var file = document.getElementById("file_upload").files[0];
            alert(file.size);
            var formdata = new FormData();
            formdata.append("filer",file,true);
            var ajax = new XMLHttpRequest();
            ajax.onreadystatechange =
            function(){
                if(ajax.readyState==4 && ajax.status==200){
                    document.getElementById("one").remove();
                    var img = document.createElement('img');
                    var first_path = '/user_image/';
                    var path = first_path.concat(id,'.png');
                    img.setAttribute('alt','User image');
                    img.setAttribute('id','one');                        
                    img.setAttribute('src',path);
                    document.getElementById("user").appendChild(img);  
                    alert("end");
                }    
                else{
                    document.getElementById("one").remove();
                    var img = document.createElement('img');
                    img.setAttribute('src','/img/loading.gif');
                    img.setAttribute('alt','User image');
                    img.setAttribute('id','one');
                    document.getElementById("user").appendChild(img);                        
                }
            }               
            ajax.open("POST","upload_image.php");
            ajax.setRequestHeader("Content-Type", "multipart/form-data");
            ajax.send(formdata);
        };

And my php code is simple is just to test if everything is ok

require("../includes/config.php"); //config folder to start the session
if($_SERVER["REQUEST_METHOD"]=="POST"){
       echo '<pre>',print_r($_FILES),'</pre>'; //dumping some variable and arrays to see where the problem is 
}

The request that i get from server is Warning: Missing boundary in multipart/form-data POST data in Unknown on line 0 but i sent the formdata and the request header and i opened the file .

Answer 1

您必须删除以下行：

ajax.setRequestHeader("Content-Type", "multipart/form-data");

Answer 2

Instead of sending the data as multipart/form-data :

ajax.setRequestHeader("Content-Type", "multipart/form-data");

You should send it as application/json :

ajax.setRequestHeader("Content-Type", "application/json");