diff --git a/boards/admin.py b/boards/admin.py
--- a/boards/admin.py
+++ b/boards/admin.py
@@ -148,9 +148,17 @@ class BannerAdmin(admin.ModelAdmin):
 
 @admin.register(Attachment)
 class AttachmentAdmin(admin.ModelAdmin):
-    list_display = ('mimetype', 'file', 'url', 'alias')
+    list_display = ('__str__', 'mimetype', 'file', 'url', 'alias')
     search_fields = ('alias',)
 
+    def delete_alias(self, request, queryset):
+        for attachment in queryset:
+            attachment.alias = None
+            attachment.save(update_fields=['alias'])
+        self.message_user(request, _('Aliases removed'))
+
+    actions = ['delete_alias']
+
 
 @admin.register(GlobalId)
 class GlobalIdAdmin(admin.ModelAdmin):
diff --git a/boards/management/commands/statistics.py b/boards/management/commands/statistics.py
--- a/boards/management/commands/statistics.py
+++ b/boards/management/commands/statistics.py
@@ -11,7 +11,7 @@ class Command(BaseCommand):
     def handle(self, *args, **options):
         print('* Domains and their usage')
         domains = {}
-        for attachment in Attachment.objects.exclude(url=None).exclude(url=''):
+        for attachment in Attachment.objects.exclude(url=''):
             full_domain = attachment.url.split('/')[2]
             domain = get_domain(full_domain)
             if domain in domains:
@@ -25,10 +25,10 @@ class Command(BaseCommand):
         print('* Overall numbers')
         print('{} attachments in the system, {} of them as URLs'.format(
             Attachment.objects.count(),
-            Attachment.objects.exclude(url=None).count()))
+            Attachment.objects.exclude(url='').count()))
 
         print('* File types')
-        mimetypes = Attachment.objects.filter(url=None)\
+        mimetypes = Attachment.objects.filter(url='')\
                 .values('mimetype').annotate(count=Count('id'))\
                 .order_by('-count')
         for mimetype in mimetypes:
diff --git a/boards/migrations/0054_auto_20161228_2244.py b/boards/migrations/0054_auto_20161228_2244.py
new file mode 100644
--- /dev/null
+++ b/boards/migrations/0054_auto_20161228_2244.py
@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.4 on 2016-12-28 20:44
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('boards', '0053_auto_20161127_1541'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='attachment',
+            name='alias',
+            field=models.TextField(null=True, unique=True),
+        ),
+        migrations.AlterField(
+            model_name='attachment',
+            name='url',
+            field=models.TextField(blank=True, default=''),
+        ),
+    ]
diff --git a/boards/models/attachment/__init__.py b/boards/models/attachment/__init__.py
--- a/boards/models/attachment/__init__.py
+++ b/boards/models/attachment/__init__.py
@@ -45,8 +45,8 @@ class Attachment(models.Model):
     file = models.FileField(upload_to=get_upload_filename, null=True)
     mimetype = models.CharField(max_length=50, null=True)
     hash = models.CharField(max_length=36, null=True)
-    alias = models.TextField(unique=True, null=True, blank=True)
-    url = models.TextField(null=True, blank=True)
+    alias = models.TextField(unique=True, null=True)
+    url = models.TextField(blank=True, default='')
 
     def get_view(self):
         file_viewer = None