Caching, MIME or Expiry Problem?

The problem I am experiencing is not directly related to PHP (or so I believe), but rather a web application problem. So I still hope to find maybe some suggestions by posting here. What we try to do: We store masses images (TIFF images) on a file server. To search for images, we have a little database index that users can search against and in which we store the paths to the images. When they found the record for image they are looking for, we serve the image via a script/link and they should be able to view it in a plug-in (AlternaTIFF, Wang Image viewer, etc). We're currenlty beta testing this application and we have mostly happy users, but some experience funny things: They have a few things in common: A) They all sit behind the same Proxy Server (MS Proxy Server 2.0). We told them how to configure the server to ignore pages from our site and we *think* this works, as they initally had problems to log in again after they timed out of their 1st log in. So we used to see lot's of Proxy error messages which now have disappeared. The only error we still see is the "time out" error and as they are on a slow connection, we assume that's what's causing it. B) They all use WinNT4 and IE5.0. The Problems: 1.) They can only see the 1st page of a multi-page TIFF file. When requesting a subsequent page, the 1st page is loaded again. Could that be the plug-in? But we have other users using the same plug-in not experiencing this (or any other) problem, so why would it work in one place and not in another? (I admit I don't understand too much about how plug-ins work.) 2.) When they have closed the plug-in viewer, then want to see the same file again, they get a blank screen, the egg timer and nothing seem to happen. We have put a trace onto the server-browser communication and it appears as if the browser (or maybe the proxy?) suddenly decides not to want the file anymore and sends a reset. Our pages all have instructions not to be cached in the HTTP header and in browser-specific META tags. The only exception is the page opening when loading the image, as the plug-in requires caching to display the image. While looking at the network conversation, we noticed that in sometimes the same browser (tested to be true for IE and Netscape) sends a long list of specific file/MIME types in the Accept-header, ending with "*\*", but in other cases the browser just sends "*\*" in that header, nothing else. Why is that not consistent? Please understand, I am talking about the same browser on a single machine behaving differently depending on what page is requested. Any suggestions that help me understand what is going here very welcomed. I am utterly confused. In the long run, I will look for a problem that will not require plug-ins, as I believe this is part of the problem. But for now, I have no choice. Any idea when PHP supports TIFF files? Cheers Marc