Thumbnail: Fix CMYK images

src/pretix/helpers/thumb.py

@@ -58,6 +58,8 @@ def create_thumbnail(sourcename, size):
     checksum = hashlib.md5(image.tobytes()).hexdigest()
     name = checksum + '.' + size.replace('^', 'c') + '.png'
     buffer = BytesIO()
+    if image.mode not in ("1", "L", "RGB"):
+        image = image.convert('RGB')
     image.save(fp=buffer, format='PNG')
     imgfile = ContentFile(buffer.getvalue())