The Problem
We all need to send an attachment now and then, but mixmaster isn't very good for that purpose. It's primarily a text messaging protocol with a max message size of around 29K. Not many file attachments meet this size restriction and to make matters worse, the required MIME encoding will increase the file's size by about 30%. Messages larger than 29K get split up into multiple messages. Everything is reassembled at the last remailer, before delivery. The problem is, the more messages, the more likely one won't make it to that last remailer. In that case, the remailer eventually discards the incomplete message.
It doesn't take a personal computer to figure out that an attachment of a mere 200K will result in roughly 9 messages being sent. The message might get through, but it might not. Say, you decide to send 2 or more copies to boost reliability. With just 2 copies the result is 18 messages, 3 copies the result is 27 messages, and so on. Possibly you also decided to keep the chain short to aid in reliability. Unfortunately, you will discover mixmaster can't create enough unique chains to cover the required number of messages. This might require you add 2, 3 or more remailers to your chain just to get the message out the door. All of this is based on a relatively small file. If you're thinking of something bigger, well...good luck.
If you get an error referring to 'duplicate chains', you will need to add another remailer to your chain. If you still get the same error, add another, etc. The problem is, mixmaster can't create enough unique remailer chains for the number required by your attachment. By adding a remailer to the chain, you give it greater possibility to find the unique chains required.
Tools|Remailer caps...
This is a tool I've provided in QS, that allows you to check the max message size each remailer will allow. Some allow large attachments. Some don't. Some have no limit at all, but don't be fooled. As described above there are practical limits. It's the final remailer in your chain that you're concerned about. It assembles all parts and handles the full size message. You can calculate the rough size of your message w/attachment and check there for eligible exit remailers. Then create your chain header accordingly. When you send the message I'll do a check and see if there's a problem with the exit remailer. If there is, I'll let you know and let you choose from a list of eligible exits. Important: QS uses the mixmaster document rlist.txt to find remailer max message size. The document should be fresh. If you've chosen bnot to update this page regularly with the other remailer pages, then you should update that page before checking Remailer caps.
PGP Encryption/Signing
Normally, QS attaches files using a conventional multi-part MIME message. In the case of a PGP encrypted or signed message the files are attached inline in the message body. This can create a problem if the receiving email program doesn't recognize these attachments. In that case, QS includes a decoding feature available at Tools/Decode.