The 'characters' that you see are simply an attempt to convert the bits that represent the image into something/anything that the browser can display. It is simply taking each set of 4 (I think) bits and using that to fetch the appropriate letter usually represented by that digital number. It then draws that letter on the screen and goes on to the next 4 (or however many) bits and does the same thing. I think as long as it finds and "end of file" marker at what would be an appropriate place in the series of bits, it can display the letters and gracefully move on to the html.
Of course, had the file included
any suffix, especially one that would have indicated that the file was an image of some sort, the browser would have been able, more than likely, to have displayed the image instead. Since there was
no suffix, the browser falls back to 'deciding' this is another of those stupid 'human-readable-only' files and blindly dumps all those characters on the screen. "They sure look screwy to me, but it must mean something to those crazy people out there!"
An aside:
This kind of display is the same thing I see if I 'preview' an email in POPmonitor that is basically an ad for almost anything. The bits for the image are just converted to characters. That's because POPmonitor is intentional designed
not to display images. Remember, emails with references to an image back on the SPAMers server which will log your address as 'alive' and suitable for adding to the list of 'valuable' ones (with free shipping, of course!). Even if the image is never actually seen by you, it may be a 1 pixel by 1 pixel, invisible ".gif" at the end of the signature or someplace else where you'd never notice the thing. It's one of the reasons many suggest turning html OFF in an email program, they may not be as 'pretty' but they
will be safe!
Whether this technique is still in use, is another matter. But I do know that some of my SPAM is nothing but an image, or rather a reference to one.
SPAMsieve usually just dumps those into the Junk folder, anyway.