If you include an image field in a Form (featured image, or a custom image field) that is not using the WP Media Uploader, when submitting the form if the image field is blank a validation error appears:
“You can only upload an image file”.
A pre-release version of Forms 2.6.9 containing the fix is available here.
This is still undergoing final testing this week, and all being well will be the version released next Monday.
Note that if there are any changes in the meantime you will need to manually replace this version with the final version available from your account downloads page when it becomes available.