Form Builder: Validation of files

[rowan-simms]

eg to check the mime type is valid, or to check the format of the file

[charlieTheBotDev]

R.e. the mime type, you should be able to specify file types using the acceptedExtensions option (e.g. ['json']) however that just 'suggests' which file types can be uploaded, however for some operating systems (ubuntu?) it doesn't prevent you from selecting files of other types from the file upload dialog.

An option for validator should be added that is run for every file when selected. It should probably be async in case the validator needs to be server side, I'd suggest a signature something like:

validator: function (file, callback) {
  var isValid = // Boolean: file is valid
  callback(file, isValid)
}