Safari seems unable to handle multi-part documents, instead downloading them to the desktop. For an example, see the URL above. You should see a picture—it works fine in Apple Mail, this should be part of WebKit, not application-specific.
Do any web browsers currently support multipart/mixed content? This would allow servers to send content and images in one stream. It would be interesting for efficiency purposes (only needed one socket connection per page), although images would load slower overall on a web page structured this way. Safari does support multipart/x-mixed-replace, which was created by Netscape. http://wp.netscape.com/assist/net_sites/pushpull.html It's used for Bugzilla's search page and by most travel web sites when searching for airplane flights.
Opera 9.0 p2 Displays a blank page with the correct document title ("A picture"). Firefox 1.5.0.4 Displays "multipart (JPEG Image)" in the title bar, and the URL of the page as the document body. (!) iCab 3.0.2 b382 While loading, the correct title is used, after loading, the URL is displayed in the title bar. A broken image icon is displayed as the document body.
I should commend iCab here actually, it seems to understand the multi-part boundaries as a source view shows, but only fails to associate cid: URLs with the Content-ID header. This would be very useful for pages that generate dynamic images which shouldn't be cached, webcams for instance could send on one connection a continuous stream of JPEGs with the same ID, or charts fast-moving data without having to use streams of XMLHttpRequests.
This is similar to Bug 7168, which needs multipart/related support to display MHTML web archives produced by MSIE.
What *I* really want this for is sending small (<120 bytes) background images along with the CSS file that specifies them, so as to cut down the number of connections per visitor, and make my log files a little briefer. Since most visitors are on screen media with background-image-supporting UAs, I don't see sending images that are likely to be wanted anyway as a problem. I would have to include Location headers with a file path, so the UA knew where the image came from and so it can be cached and used in other documents. The example at the URL above only uses the Content-ID MIME header, and cannot thus be cached.
See also Bug 16276.
I think this is mostly a CFNetwork bug and thus should be moved to radar and closed in bugzilla.
*** Bug 16276 has been marked as a duplicate of this bug. ***
<rdar://problem/5654367>