I seem to be using the following:
upload_file:trim,notnull
upload_file.tmpfile:tmpfile
....
# then in the body of the tcl page:
set file_id [open ${upload_file.tmpfile} r]
...
but I needed to read using ns_getcsv, so if you want to copy it just use 'file copy ${upload_file.tmpfile} newfilename'