Persuading an email client not to display an html attachment inline?

[ALMOBDE3]

Problem: I send out an email (using phpmailer and sendmail) which has a complex html attachment. I don't want the email client to attempt to display the attachment, since it's far too complex to be viewed in a mail reader.As far as I can make out, the way to do this is to set \[code\]content-Disposition\[/code\] to \[code\]attachment\[/code\]. The default in phpmailer is to set it to \[code\]attachment\[/code\] anyway. However, Thunderbird (the only client I've tested so far) does actually attempt to display the html. If I view the headers in Thunderbird, I can't see a \[code\]Content-Disposition\[/code\], but if I save the mail I can see:\[code\]--b1_420d367a26870bbc3bae73fb1de31f49Content-Type: application/octet-stream; name="chart.html"Content-Transfer-Encoding: base64Content-Disposition: attachment; filename="chart.html"\[/code\]Thunderbird has given the attachment the correct name, so it presumably understands the \[code\]Content-Disposition\[/code\]. Is there something I can do with phpmailer to stop clients displaying html attachments, or can clients just do what they want?Sending a compressed file isn't really an option - the users may not be sophisticated enough to know what to do with it.