If you look at the source code of PHP, you can see on the file php-5.4.8-srcmain fc1867.c line 706-709 this:

if (SG(post_max_size) > 0 && SG(request_info).content_length > SG(post_max_size)) {
    sapi_module.sapi_error(E_WARNING, "POST Content-Length of %ld bytes exceeds the limit of %ld bytes", SG(request_info).content_length, SG(post_max_size));
    return;
}

Same is there also in file php-5.4.8-srcmainSAPI.c. So, the message PHP Warning: POST Content-Length of 12221448 bytes exceeds the limit of 8388608 bytes in Unknown on line 0 is about post_max_size setting. You have confirmed from using phpinfo() that you have this setting configured correctly, but it seems to be using the default value of 8M anyway.

As to why, see this thread:

As it turns out, on Windows, you can only set ini directives that are marked PHP_INI_USER per directory. Unfortunately, upload_max_filesize and post_max_size are both PHP_INI_PERDIR. From the PHP docs at http://php.net/manual/en/configuration.changes.php

The settings for the directory would be active for any script running from this directory or any subdirectory of it. The values under the key should have the name of the PHP configuration directive and the string value. PHP constants in the values are not parsed. However, only configuration values changeable in PHP_INI_USER can be set this way, PHP_INI_PERDIR values can not.

So even though Plesk has an interface to change those directives, and even though phpinfo() picks up on them, they do nothing to change the actual max upload sizes. Plesk should not allow you to change those on Windows, and phpinfo() should not report the change, but what can you do.

So, it's post_max_size, and it needs to be set on php.ini. Plesk setting simply will not work, even though phpinfo says otherwise. I also opened a bug entry on phpinfo behaviour as there didn't seem to be an entry for it.