diff --git a/bookwyrm/models/fields.py b/bookwyrm/models/fields.py
index caa22fcd..379323b9 100644
--- a/bookwyrm/models/fields.py
+++ b/bookwyrm/models/fields.py
@@ -1,5 +1,6 @@
 """ activitypub-aware django model fields """
 from dataclasses import MISSING
+import imghdr
 import re
 from uuid import uuid4
 
@@ -9,7 +10,7 @@ from django.contrib.postgres.fields import ArrayField as DjangoArrayField
 from django.core.exceptions import ValidationError
 from django.core.files.base import ContentFile
 from django.db import models
-from django.forms import ClearableFileInput, ImageField
+from django.forms import ClearableFileInput, ImageField as DjangoImageField
 from django.utils import timezone
 from django.utils.translation import gettext_lazy as _
 from bookwyrm import activitypub
@@ -334,10 +335,14 @@ class TagField(ManyToManyField):
 
 
 class ClearableFileInputWithWarning(ClearableFileInput):
+    """max file size warning"""
+
     template_name = "widgets/clearable_file_input_with_warning.html"
 
 
-class CustomImageField(ImageField):
+class CustomImageField(DjangoImageField):
+    """overwrites image field for form"""
+
     widget = ClearableFileInputWithWarning
 
 
@@ -400,11 +405,12 @@ class ImageField(ActivitypubFieldMixin, models.ImageField):
         if not response:
             return None
 
-        image_name = str(uuid4()) + "." + url.split(".")[-1]
         image_content = ContentFile(response.content)
+        image_name = str(uuid4()) + "." + imghdr.what(None, image_content.read())
         return [image_name, image_content]
 
     def formfield(self, **kwargs):
+        """special case for forms"""
         return super().formfield(
             **{
                 "form_class": CustomImageField,