Allow files with upper case extensions when uploading avatar

Discovered when attempting to upload a file with name avatar.JPG

The following error was reported in the logs:
```
File "/app/funkwhale_api/common/serializers.py", line 213, in to_internal_value
     format=PIL.Image.EXTENSION[os.path.splitext(file_obj.name)[-1]],
 KeyError: '.JPG'
```

api/funkwhale_api/common/serializers.py

@@ -219,7 +219,7 @@ class StripExifImageField(serializers.ImageField):
         with io.BytesIO() as output:
             image_without_exif.save(
                 output,
-                format=PIL.Image.EXTENSION[os.path.splitext(file_obj.name)[-1]],
+                format=PIL.Image.EXTENSION[os.path.splitext(file_obj.name)[-1].lower()],
                 quality=100,
             )
             content = output.getvalue()