Move extension preservetation code to the model. Improve document,

version and API views to include proper mimetype and encoding.
Update respective tests. GitLab issue #415.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/views/document_views.py

@@ -7,7 +7,6 @@ from django.core.exceptions import PermissionDenied
 from django.http import HttpResponseRedirect
 from django.shortcuts import get_object_or_404
 from django.urls import reverse, reverse_lazy
-from django.utils.encoding import force_text
 from django.utils.http import urlencode
 from django.utils.translation import ugettext_lazy as _, ungettext