Multiple file upload doesn't work in Safari 6.1 above, unless web inspector open

So I have a file upload site, which I develop using HTML5 chunking ability to upload multiple files. it works fine on Chrome, Firefox, IE (basicly browser with HTML5 capability) as well as Safari, but recently I test it out, Safari 6.0.5 works fine, but on Safari 6.1, If I upload multiple files, some files are 0 bytes. I'm not sure what happened.

When I tested, I upload about 70 files totalling 200MB, and each file is between 5-8MBish.. so there's no chunking happenening.. but when I check on the server, most file are 0 bytes (like it never get uploaded) except a few files (probably 3-5 files)

Is there any difference between Safari 6.0.5 and below, with Safari 6.1?

My code is basicly in a nutshell: Javascript will chunk each file if it's bigger than 10MB/file, if not it will just upload as is. then PHP will handle the upload (standard file upload style move_uploaded_file()).

function uploadFile(file_blob_chunk, file_name, file_part, total_file_chunk, file_id) {
    //create a progress bar based on file id (check if it's the 0 part, otherwise there will be multiple bar for same file)
    if(file_part == 0) {
        progressBar(file_id);
    }

    //ajax call for creating multipart data form
    fd = new FormData();    
    fd.append("file_for_upload", file_blob_chunk);
    fd.append("file_id", file_id);
    fd.append("file_name", file_name);
    fd.append("file_part", file_part);

    xhr = new XMLHttpRequest();
    xhr.fid = file_id;
    xhr.fid_name = file_name;
    xhr.fid_part = file_part;
    xhr.fid_total_chunk = total_file_chunk; 

    xhr.upload.fid = file_id;
    xhr.upload.fid_part = file_part;
    xhr.upload.fid_total_chunk = total_file_chunk;
    xhr.open("POST", "datas/upload/" + file_name + '/' + file_part, true);

    xhr.send(fd);

code wise it's something like that...

any idea what's wrong with safari 6.1?

I check the tmp folder, the tmp file during upload is 0 bytes..

NOTE: Safari 6.1+, If web inspector on, every file is uploaded correctly, if it's off, out of 10 files, only 3 got uploaded the rest is 0 bytes. what cause this difference?