Thanks for answering my question quickly, but I have already read the post you linked as an answer and also tried that.
1) my value for
max_input_vars was 1000 and I have increased it to 10000 (before writing the question) checked using phpinfo command.
2) In my opinion Its not about max_input_vars because then I would have not seen the values of all (6) media files in $_FILES array populated after submit
Do you have any other suggestions.
Thanks & Regards,