You can get 'Undefined class' error message if you try to create a record for a class that does not exist.
Please check if you have such a class in the Admin panel in the “Custom” section of your application. If not, please create a new class with such name.
Also, the issue can be caused by absence of the file extension you try to upload on a list of allowed extensions for Enterprise server.
You can find the list here: What file extensions are allowed?
Please contact our Support team in order to add more allowed extension to the list.
Feel free to check our official documentation for more information that will help you to integrate QuickBlox services into your applications / projects or contact our Support team by submitting a ticket if you have any other questions.