If you try to post an un-validated form, when you re-render the form, the filefield will be empty.
See this issue for more explanation..
Django Form File Field disappears on form error
The following works for me. Add a clear button to your html
<input type="submit" name = "submit-clear" value="clear file">
Then in your view handle this button using..
if 'submit-clear' in request.POST:
return HttpResponseRedirect('')
This should clear any filefields but will also remove any other settings the user has made, if you want to preserve other settings and just remove the filefield, then ensure the form does not validate somehow so that it renders again.