Attachments are very bad for the mixmaster network. It is intended to move text messages with stealth and security. A primary attribute of the system is that all messages are approximately 29K in size. This is the case whether your message is a single 50 byte sentence or a 100K detailed document. So how does a 100K message get squeezed into a 29K packet? The answer is simple. It doesn't. It breaks the messages up into a number of smaller parts what will fit into the packet payload area. This means that instead of a single packet being sent when you click send, the number of packets required will be sent--each with it;s own chain and each ending at the same remailer. That remailer reassembles the message when all parts have arrived and delivers it as a whole to the final recipient.
This process can increase the load on the network dramatically. For example, I sent out a message with a 116K attachment. I used a chain of 4 remailers and chose 3 copies to help insure successful delivery. That all seemed ordinary enough. A 116K attachment is really, really small by todays standards. The surprise came when mixmaster encrypted the message. The result was 30 outgoing messages! Just for my small 116K attachment. Also, the chances the message won't be delivered are pretty good, so you mayy need to send the attachment again.
As I said, attachments are bad for the network. We all need to send one once in a while. Please keep it small and infrequent.
One of the most commonly encountered problem is mixmaster not being able to create enough unique chains to cover the number of packets to be sent. In this case you'll see QS report "I've encountered a problem with remailer selections. No big deal! Starting over!" The problem is, QS tries and tries until finally giving up. The only possible fix for this problem is to add a remailer to your chain. If the problem still exists, another must be added, and so on. Of course, with each remailer added to the chain, the odds of the message being delivered drop some. You may need to increase copies to aid reliability, but that will create many more packets to be dealt with and still your mail may not make it to the recipient.