I would very much like to be able to return pictures including Postscript. I know this has been discussed in the past. One issue is how will the client know what kind of data is being returned. Simplest, but makes the client do too much work. A server which wishes to return non-text data first returns a constructed document containing anchors where the link type somehow encodes the data type.
Benefit here is that it allows the user to choose the kind of data if there is a choice.
Servers will probabaly construct a new document for each query, using MIME's external reference feature where possible to avoid copying large data files.