Hi Catweazle,
Obviously I can't say what is the reason of file truncation, however I can say that, when I developed the first versions of HttpSvr using the official Ethernet library and W5100 driver, I ran under the impression that something in the sequence of operations for reading incoming bytes was somehow obscure or not always predictable. This is why I decided to rewrite the whole stuff from scratch bypassing the Ethernet library - just to have full awareness of what was going on. Of course I don't mean that the library doesn't work, only that I could not find exactly what was wrong in my way of using it for file upload.
Anyway, the unsuccessful experiments of long file uploading made with the official library seemed to work successfully with my homemade version (up to several MB - beware, it takes long), so I didn't go any further in investigating the Ethernet library.
In my code I tried to put very special care in clearing W5100 flags and moving counters as prescribed in the datasheet, and it seems to work better for me.
Last but not least, I am not putting on HttpSvr project professional resources, so I did not perform serious test campaigns, nor am I in the condition of offering professional support to the project. However, if you are interested in having a look at the sources, you can find them at Google Code Archive - Long-term storage for Google Code Project Hosting..
HTH,
Enrico