Your server configuration settings allows users to upload files upto 16MB (because you have set upload_max_filesize = 16Mb) but the post_max_size accepts post data upto 8MB only. This is why it throws an error.
Quoted from the official PHP site:
To upload large files, post_max_size value must be larger than upload_max_filesize.
memory_limit should be larger than post_max_size
You should always set your post_max_size value greater than the upload_max_filesize value.