By default, PHP limits uploaded files size to 2 MB, and this limit applies to all PHP application including Webmail. Edit the following options in /etc/php.ini file on the Plesk server and set the values that match your needs, for example:
upload_max_filesize = 32M
post_max_size = 32M
Restart Apache after the php.ini file modification.
Also, maximum attached file size can be changed in Horde configuration. See “attach_size_limit” option in the /etc/psa-horde/imp/conf.php:
$conf[‘compose’][‘attach_size_limit’] = ‘0’;
Zero “attach_size_limit” value means ‘no limit’.
Note that memory_limit should be larger than post_max_size.
If you insrease PHP parameter post_max_size do not forget to increase parameter memory_limit also.