Multer not accepting files in array format gives 'Unexpected File Error'

The reason for the error is that multer currently does not support the array syntax that ng-file-upload uses by default which is files[0], files[1], files[2], etc. multer is expecting a series of files with the same field name.

The easiest solution is to set ng-file-upload's arrayKey option like so to avoid appending the [index] part:

Upload.upload({  url: '/api/data/addtweet',  arrayKey: '', // default is '[i]'  data: {    files: files  }})

If someone is facing a similar issue when uploading a custom form data object, you can use this approach. In here I am not using ngFileUpload.

Client Side code

var fd = new FormData();for( var i =0; i< files.length ; i++ ){    fd.append('fileItems' , files[i] );}fd.append('_id', params._id );              fd.append('user', params.user );              return $http.post( ROOT_URL + '/uploadFiles/', fd, {    transformRequest: angular.identity,    headers: {'Content-Type': undefined }});

Express Router

var multer  = require("multer");var upload = multer({ dest: "uploads/" });app.post('/api/uploadFiles', upload.array('fileItems'), handlers.files.uploadFiles);