Ah, my bad: actually, the chance is zero, as <form enctype="plain/text" method="post"> would still post parameter names (as given in <input name="whatever">) along with their values, not only the value(s).
That’s the result of the browser trying to find an icon after navigating to the action URL while posting the data. You’ll want to see what the response of the POST to that action URL is. Make sure to use something like “Persist logs” or “Preserve logs” in your browser’s network tab, to see the result of that very URL.