Update: I just got around to trying webupload.arc, and like you said it can't handle that 800KB bzip2 upload from localhost. Files larger than 99339 bytes only render the first 99339 bytes.

I modified /upload to not print the entire file. Somewhere just before 400k the upload dies, again like you said. I experimented with various file sizes. At 392080 bytes, it always succeeds, at 392100 bytes it always fails. At 392090 bytes it fails around half the time. At 392085 bytes it fails around a third of the time.

Very weird.

I think the next step is to build an upload form in pure racket, and see if it suffers from the same problem. I'm not sure when I'll be able to get around to it; it'll require some reading of racket docs.

Update 2: curl indicates both commands behave the same way.

  $ curl -i -F "name=@x.100k" "http://localhost:8080/x?fnid=7Uakm57k8V"
  HTTP/1.1 200 OK
  Content-Type: text/html; charset=utf-8
  Connection: close

   ... headers ...
  <br>Content-Type: multipart/form-data; boundary=----------------------------fd366e3dc6eb
  <br>Content-Length: 100198
  <br> ... more headers ...
  <br><br>
  100198

  $ curl -i -F "name=@x.392100" "http://localhost:8080/x?fnid=7Uakm57k8V"
  HTTP/1.1 200 OK
  Content-Type: text/html; charset=utf-8
  Connection: close

   ... headers ...
  <br>Content-Type: multipart/form-data; boundary=----------------------------c97bd48c8631
  <br>Content-Length: 392300
  <br> ... more headers ...
  <br><br>
  392300