File transfer upload weird problem!

Hi, I want upload a file to server, so I get file path and file sended very good(I get responseCode: 200). but in backend (php), $_FILES is empty! how it possible?
note : when I send a form data and try again for upload this file it worked! :confused: Is something like cache?
code :

 let options: FileUploadOptions = {
        fileKey: 'files',
        fileName: fileName.substr(fileName.lastIndexOf('/') + 1),
        mimeType: 'multipart/form-data',
        chunkedMode: false,
        params: obj,
        headers: this.headers


this.fileTransfer.upload(fileName, encodeURI(link), options)
        .then((data) => {
          this.messageCtrlService.showAlert(JSON.stringify(data), 'Success');
          return data.response;

        }, (err) => {
          this.messageCtrlService.showAlert(JSON.stringify(err), 'Error');