In the main mail listview where all your mails in a folder are listed, the Attachment status icon (normally a paper-clip icon) is sometimes shown, but suddenly disappears as soon as you click on that mail for viewing its content. Even if that may look a bit strange and may lead to the impression that attachments are being lost when viewing the mail, this behaviour is normal and correct.

The reason why this is happening is that unfortunately some broken (old) mail clients tend to send out emails where the main content-type is set to "multipart/mixed" or "multipart/related" even if that email just contains one main mail text part without a real attachment. This is clearly incorrect according to the RFCs and can be considered a bug in those email programs. However, the reason why YAM is first showing the attachment status icon is that it just analyzes the main mail header due to performance reasons and as soon as you click on the message a depper (more time consuming) analyze is performed where it automatically recognizes that the mail doesn't really contain any binary or text attachment, but just the main normal mail text.

So the behaviour of YAM is fully correct and in line what the RFCs suggests.