I am trying to upload a test file that is 2.2 MB. I tried with and without the chunked file upload and it keeps giving me this error. I have a simple for with only a few fields here:
http://www.cohesivewebdesigns.com/purchase-pdf-flipbook-conversion/ (still under production so ignore the SSL credit card warning)
I did not have anything enabled on the plugin such as drag and drop and it still gave me that error. We are on WP Engine's servers and I think they have some special kind of server side caching so not sure what is going on with it.