I had problem with uploading files on Directadmin server with mod_security enabled. I set upload_max_filesize and post_max_size to 64M, and still recived error like this:
[Wed Aug 04 23:31:30.320425 1912] [:error] [pid 1418987:tid 140666332325632] [client 126.96.36.199:64983] [client 188.8.131.52] ModSecurity: Request body (Content-Length) is larger than the configured limit (13107200). [hostname "www.geekytuts.net"] [uri "/wp-admin/async-upload.php"] [unique_id "YQpCUpEZgUlp9rxIlbunrAAAnTQ"], referer: https://www.geekytuts.net/wp-admin/media-new.php
I found out that this behavior is not caused by any mod_security rules (Free ModSecurity Rules from Comodo) but by directive SecRequestBodyLimit setting . Default value is 12.5M and since I set 64M for file uploads, I changed this value the same – in bytes.
File which you want to modify is /etc/httpd/conf/extra/httpd-modsecurity.conf. In my case, I modifyed like this:
SecRequestBodyLimit 67108864 SecRequestBodyNoFilesLimit 1048576