This plugin can be used to upload and download files.

Installing File Transfer Plugin:

To install this plugin into your application, run the following command:
 

cordova plugin add cordova-plugin-file-transfer

After installing the plugin, you need to create two buttons:

<button id = "uploadFile">Upload file</button>
<button id = "downloadFile">Download file</button>

Next, you need to add click events and callback functions on these buttons inside device ready:

document.getElementById("uploadFile").addEventListener("click", uploadFile);
document.getElementById("downloadFile").addEventListener("click", downloadFile);

Download function:

To download the file from server into device, we will create this function. It is defined in js file, when you click on download button, it is called.

function downloadFile() {

   var fileTransfer = new FileTransfer();
   var uri = encodeURI("http://s14.postimg.org/i8qvaxyup/bitcoin1.jpg");
   var fileURL =  "///storage/emulated/0/DCIM/myFile";

   fileTransfer.download(
      uri, fileURL, function(entry) {
         console.log("download complete: " + entry.toURL());
      },
        
      function(error) {
         console.log("download error source " + error.source);
         console.log("download error target " + error.target);
         console.log("download error code" + error.code);
      },
        
      false, {
         headers: {
            "Authorization": "Basic dGVzdHVzZXJuYW1lOnRlc3RwYXNzd29yZA=="
         }
      }
   );
}

here, uri is the download link from server and fileURI is file path to save into your device.

Upload Function:

upload function will upload file to the server. this function will be called when you click on the upload file button.

function uploadFile() {
   var fileURL = "///storage/emulated/0/DCIM/myFile"
   var uri = encodeURI("http://posttestserver.com/post.php");
   var options = new FileUploadOptions();
    
   options.fileKey = "file";
   options.fileName = fileURL.substr(fileURL.lastIndexOf('/')+1);
   options.mimeType = "text/plain";

   var headers = {'headerParam':'headerValue'};
   options.headers = headers;

   var ft = new FileTransfer();

   ft.upload(fileURL, uri, onSuccess, onError, options);

   function onSuccess(r) {
      console.log("Code = " + r.responseCode);
      console.log("Response = " + r.response);
      console.log("Sent = " + r.bytesSent);
   }

   function onError(error) {
      alert("An error has occurred: Code = " + error.code);
      console.log("upload error source " + error.source);
      console.log("upload error target " + error.target);
   }
    
}