Solution:1
Try setting the size limit in bytes – the output for 20mb in bytes is 20971520
[file file limit:20971520 filetypes:txt|pdf|doc|docx|odt class:fileinput]Solution:2
To allow files bigger than 1Mb(?), I had to edit
wp-content/plugins/contact-form-7/modules/file.php
and changes 1048576 to 10048576
$allowed_size = 10048576;
This will allow ContactForm7 to accept files until 10Mb files, if we have the server configured to it.