diff --git a/nummi/main/migrations/0004_alter_category_options_category_full_name.py b/nummi/main/migrations/0004_alter_category_options_category_full_name.py
index 931edf6..1af43dd 100644
--- a/nummi/main/migrations/0004_alter_category_options_category_full_name.py
+++ b/nummi/main/migrations/0004_alter_category_options_category_full_name.py
@@ -6,18 +6,18 @@ from django.db import migrations, models
 class Migration(migrations.Migration):
 
     dependencies = [
-        ('main', '0003_category_parent_alter_category_name'),
+        ("main", "0003_category_parent_alter_category_name"),
     ]
 
     operations = [
         migrations.AlterModelOptions(
-            name='category',
-            options={'ordering': ['-parent_id', 'name']},
+            name="category",
+            options={"ordering": ["-parent_id", "name"]},
         ),
         migrations.AddField(
-            model_name='category',
-            name='full_name',
-            field=models.CharField(default='', max_length=512),
+            model_name="category",
+            name="full_name",
+            field=models.CharField(default="", max_length=512),
             preserve_default=False,
         ),
     ]
diff --git a/nummi/main/migrations/0005_alter_category_full_name.py b/nummi/main/migrations/0005_alter_category_full_name.py
index c52a980..af9449f 100644
--- a/nummi/main/migrations/0005_alter_category_full_name.py
+++ b/nummi/main/migrations/0005_alter_category_full_name.py
@@ -6,13 +6,13 @@ from django.db import migrations, models
 class Migration(migrations.Migration):
 
     dependencies = [
-        ('main', '0004_alter_category_options_category_full_name'),
+        ("main", "0004_alter_category_options_category_full_name"),
     ]
 
     operations = [
         migrations.AlterField(
-            model_name='category',
-            name='full_name',
-            field=models.CharField(default='', editable=False, max_length=512),
+            model_name="category",
+            name="full_name",
+            field=models.CharField(default="", editable=False, max_length=512),
         ),
     ]
diff --git a/nummi/main/models.py b/nummi/main/models.py
index a218d1a..13b9c82 100644
--- a/nummi/main/models.py
+++ b/nummi/main/models.py
@@ -1,16 +1,18 @@
 import uuid
 from django.db import models
 from django.forms import ModelForm
+from django.core.validators import validate_unicode_slug, FileExtensionValidator
 
 
 class Category(models.Model):
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
-    name = models.CharField(max_length=64)
+    name = models.CharField(max_length=64, validators=[validate_unicode_slug])
     full_name = models.CharField(max_length=512, editable=False, default="")
     parent = models.ForeignKey("self", on_delete=models.SET_NULL, blank=True, null=True)
 
     def __str__(self):
-        if self.parent is None: return self.name
+        if self.parent is None:
+            return self.name
         return f"{self.parent}>{self.name}"
 
     def save(self, *args, **kwargs):
@@ -20,7 +22,7 @@ class Category(models.Model):
             child.save()
 
     class Meta:
-        ordering = ['full_name']
+        ordering = ["full_name"]
 
 
 class Transaction(models.Model):
@@ -49,7 +51,9 @@ class TransactionForm(ModelForm):
 class Invoice(models.Model):
     id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
     name = models.CharField(max_length=256)
-    file = models.FileField(upload_to="invoices/")
+    file = models.FileField(
+        upload_to="invoices/", validators=[FileExtensionValidator(["pdf"])]
+    )
     transaction = models.ForeignKey(Transaction, on_delete=models.CASCADE)
 
     def __str__(self):
diff --git a/nummi/main/views.py b/nummi/main/views.py
index 8d20f80..95bef5e 100644
--- a/nummi/main/views.py
+++ b/nummi/main/views.py
@@ -15,7 +15,8 @@ def index(request):
         children = []
         for child in Category.objects.filter(parent=cat):
             children += _cat_list(child)
-        if len(children) == 0: return cat.name, 
+        if len(children) == 0:
+            return (cat.name,)
         return cat.name, children
 
     _cats = []