Remove extra steps to sprite assignment when using the PreviewField attribute.

Issue #372 new
Alex Slavski created an issue

If I have a Sprite field without [PreviewField] I can just drag and drop sprite file from Project field - and it will automatically assign its child element - but if I use [PreviewField] for a nice and large image preview it will not accept drag and drop from this element but will require me to either use Select button on the inspector image field or drag the child element which is counter intuitive, because without the [PreviewField] that Sprite field was still enhanced by Odin.