So my upload problem was caused by an SSL configuration issue. I had forced all http traffic on the server to use port 443. What I didn't think about is what internet site would be used to communicate between my browser and the server. It was the 'default' internet site, and the document controlling it had an incorrect key file named in it. So, when my browser went to upload the attachment, it was forced to use SSL but the SSL session failed.
It would be nice if filesendr responded without continuing to save the document and send announcement emails as if everything was OK, but once I fixed the key file reference (and restarted http), everything started working correctly.