"1. The data as it comes into the web server is Content-Type: application/octet-stream"

No, it doesn't. You're not looking at the real HTTP headers. A multipart/form-data HTTP message has an uber-header at the top, and then each form field has its own set of mini-headers.

The Content-Type: application/octet-stream header you posted is from the file field's mini-headers. The uber-headers will say Content-Type: multipart/form-data.

A full multipart/form-data HTTP request might look something like this:

POST /handler.cgi HTTP/1.1
Host: http://www.example.com/
Content-Type: multipart/form-data; boundary=XYZ

--XYZ
Content-Disposition: form-data; name="title"

Picture of Camel
--XYZ
Content-Disposition: form-data; name="upload"
Content-Type: image/jpeg

... some binary data ...
--XYZ--
[download]

The kind of form that might result in that request is:

<form action="http://www.example.com/handler.cgi"
      method="post"
      enctype="multipart/form-data">
  <fieldset>
    <legend>Image upload</legend>
    <label>Image <input name="title"></label><br>
    <label>File <input name="upload" type="file"></label>
    <input type="submit">
  </fieldset>
</form>
[download]

And my advice to check the return value of open() still stands.