Deleting all created images for the image without extension fails

[IKarbowiak]

When trying to delete all created images with the use of delete_all_created_images method I get the AssertionError.
I find out that the sized images are created with extension jpg when the base image is without extension. It causes an error during deletion as the algorithm is looking for a file with the same extension as the base one.

Steps to reproduce:

1. Save in VersatileImageField file without extension in the name.
2. Create some thumbnails.
3. Try to remove all created images with the use of delete_all_created_images.
4. You will get the AssertionError.