Tnx for your proposal SKaero,
But these options are like that already defined.
// attachments of these types will open in a preview window
// either a comma-separated list or an array: 'text/plain,text/html,text/xml,image/jpeg,image/gif,image/png,application/pdf'
$rcmail_config['client_mimetypes'] = null; # null == default
// Path to a local mime magic database file for PHPs finfo extension.
// Set to null if the default path should be used.
$rcmail_config['mime_magic'] = null;
// Absolute path to a local mime.types mapping table file.
// This is used to derive mime-types from the filename extension or vice versa.
// Such a file is usually part of the apache webserver. If you don't find a file named mime.types on your system,
// download it from http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types
$rcmail_config['mime_types'] = null;