Thanks for the help, but I did have the FileUpload script enabled. This morning I discovered the problem. Because I have been trying to display an iFrame in a separate procedure in my app, I had changed the X-Frame-Options setting (Security tab) to blank. This setting apparently makes a form with a FileUpload field non-functional. When I changed the X-Frame-Options setting back to 'same-origin', the form with the FileUpload field behaved normally.
Mark