from __future__ import unicode_literals

import os.path

from taggit.managers import TaggableManager

from django.db import models
from django.db.models.signals import pre_delete
from django.dispatch.dispatcher import receiver
from django.dispatch import Signal
from django.core.urlresolvers import reverse
from django.conf import settings
from django.utils.translation import ugettext_lazy as _
from django.utils.encoding import python_2_unicode_compatible

from wagtail.wagtailadmin.taggable import TagSearchable
from wagtail.wagtailadmin.utils import get_object_usage
from wagtail.wagtailsearch import index


@python_2_unicode_compatible
class Document(models.Model, TagSearchable):
    title = models.CharField(max_length=255, verbose_name=_('Title'))
    file = models.FileField(upload_to='documents', verbose_name=_('File'))
    created_at = models.DateTimeField(verbose_name=_('Created at'), auto_now_add=True)
    uploaded_by_user = models.ForeignKey(settings.AUTH_USER_MODEL, verbose_name=_('Uploaded by user'), null=True, blank=True, editable=False)

    tags = TaggableManager(help_text=None, blank=True, verbose_name=_('Tags'))

    search_fields = TagSearchable.search_fields + (
        index.FilterField('uploaded_by_user'),
    )

    def __str__(self):
        return self.title

    @property
    def filename(self):
        return os.path.basename(self.file.name)

    @property
    def file_extension(self):
        parts = self.filename.split('.')
        if len(parts) > 1:
            return parts[-1]
        else:
            return ''

    @property
    def url(self):
        return reverse('wagtaildocs_serve', args=[self.id, self.filename])

    def get_usage(self):
        return get_object_usage(self)

    @property
    def usage_url(self):
        return reverse('wagtaildocs:document_usage',
                       args=(self.id,))

    def is_editable_by_user(self, user):
        if user.has_perm('wagtaildocs.change_document'):
            # user has global permission to change documents
            return True
        elif user.has_perm('wagtaildocs.add_document') and self.uploaded_by_user == user:
            # user has document add permission, which also implicitly provides permission to edit their own documents
            return True
        else:
            return False

    class Meta:
        verbose_name = _('Document')


# Receive the pre_delete signal and delete the file associated with the model instance.
@receiver(pre_delete, sender=Document)
def document_delete(sender, instance, **kwargs):
    # Pass false so FileField doesn't save the model.
    instance.file.delete(False)


document_served = Signal(providing_args=['request'])

from __future__ import unicode_literals

import os.path

from taggit.managers import TaggableManager

from django.db import models
from django.db.models.signals import pre_delete
from django.dispatch.dispatcher import receiver
from django.dispatch import Signal
from django.core.urlresolvers import reverse
from django.conf import settings
from django.utils.translation import ugettext_lazy as _
from django.utils.encoding import python_2_unicode_compatible

from wagtail.wagtailadmin.taggable import TagSearchable
from wagtail.wagtailadmin.utils import get_object_usage
from wagtail.wagtailsearch import index


@python_2_unicode_compatible
class Document(models.Model, TagSearchable):
    title = models.CharField(max_length=255, verbose_name=_('Title'))
    file = models.FileField(upload_to='documents', verbose_name=_('File'))
    created_at = models.DateTimeField(verbose_name=_('Created at'), auto_now_add=True)
    uploaded_by_user = models.ForeignKey(settings.AUTH_USER_MODEL, verbose_name=_('Uploaded by user'), null=True, blank=True, editable=False)

    tags = TaggableManager(help_text=None, blank=True, verbose_name=_('Tags'))

    search_fields = TagSearchable.search_fields + (
        index.FilterField('uploaded_by_user'),
    )

    def __str__(self):
        return self.title

    @property
    def filename(self):
        return os.path.basename(self.file.name)

    @property
    def file_extension(self):
        return os.path.splitext(self.filename)[1][1:]

    @property
    def url(self):
        return reverse('wagtaildocs_serve', args=[self.id, self.filename])

    def get_usage(self):
        return get_object_usage(self)

    @property
    def usage_url(self):
        return reverse('wagtaildocs:document_usage',
                       args=(self.id,))

    def is_editable_by_user(self, user):
        if user.has_perm('wagtaildocs.change_document'):
            # user has global permission to change documents
            return True
        elif user.has_perm('wagtaildocs.add_document') and self.uploaded_by_user == user:
            # user has document add permission, which also implicitly provides permission to edit their own documents
            return True
        else:
            return False

    class Meta:
        verbose_name = _('Document')


# Receive the pre_delete signal and delete the file associated with the model instance.
@receiver(pre_delete, sender=Document)
def document_delete(sender, instance, **kwargs):
    # Pass false so FileField doesn't save the model.
    instance.file.delete(False)


document_served = Signal(providing_args=['request'])

Use os.path.splitext() instead of custom logic