This extra parameter is available for HTTP upload since 2.5.
It allows discarding retry attempts (when using retry parameter) when "errorheader" extra parameter is set from server-side. For instance, if you have a "disk full" issue, then it isn't worth to retry upload.
<PARAM NAME="retry" VALUE="3">
<PARAM NAME="param4" VALUE="errorheader">
<PARAM NAME="value4" VALUE="custommessage">
<PARAM NAME="param5" VALUE="retryonerrorheader">
<PARAM NAME="value5" VALUE="false">
Just pass "custommessage"="Some error to display in applet" HTTP header in script on server-side and the applet will discard retry on HTTP 405 response.