Accoring to http://www.php.net/manual/en/function.imap-mail-compose.php:
If I want to add a "Content-disposition" line to an attachment, I should do something
like this:

$part['disposition.type'] = 'attachment';
$part['disposition'] = array ('filename'=>'file.txt');

But this does not work.

So I looked in the source, file ext/imap/php_imap.c, line 2998:
if (zend_hash_find(Z_ARRVAL_PP(data), "Z_TYPE(disposition)",
    sizeof("Z_TYPE(disposition)"), (void **) &pvalue)== SUCCESS) {

... and tried a work-around:

$part['Z_TYPE(disposition)'] = 'attachment';
$part['disposition'] = array ('filename'=>'file.txt');

... which works fine.

Cound someone please repair the source or the Manual page?