The maximum upload size is controlled at the server-level, not by WordPress or Contact Form 7. Here are three ways you can increase the upload limit:
1. If you can edit or override the system php.ini file, increase the maximum file and post sizes. For example, upload_max_filesize = 100M ;
and post_max_size = 100M ;
2. If you cannot edit or override the system php.ini file, add php_value upload_max_filesize 100M
and php_value post_max_size = 100M
to your .htaccess file.
3. If neither of these work, it’s time to ask your hosting provider to increase the maximum file and post sizes on your account. Keep in mind that most decent hosting providers allow this, and If your hosting provider won’t accommodate you, perhaps it’s time to find a new hosting provider.
(in the above examples, the limit is set to 100MB)