My filename changes after the upload. What it is the way to get the new filename from Javascript API?

We convert filename to a valid format. If your filename contains unacceptable symbols they will be stripped. Valid symbols include  ASCII letters, digits, "_" symbol and a proper file extension. 

If your backend needs to know what the new filename is after the upload checkout  this snippet in python.

For client side, this Javascript function will do the same: 

function cleanFileName(your_string) {
  //check for empty string
  if (your_string.length == 0)
    return false;
  var ext_re = /[^.]+$/;
  // ascii letters, digits, '_' symbol are only valid characters
  var replace_re = /[^\w^\d^\\_]/g;
  var extension = ext_re.exec(your_string)[0];
  // if filename doesn't have an extension
  if (extension.length == your_string.length)
    return your_string.replace(replace_re, "");
  // clean root and extension
  var filename_length = your_string.length;
  var extension_length = extension.length;
  var root_length = filename_length - extension_length - 1;
  var root = your_string.substring(0, root_length);
  var clean_root = root.replace(replace_re, "");
  var clean_extension = extension.replace(replace_re, "");
  return clean_root + '.' + clean_extension;