Media image field machine name should not be hardcoded

My site uses "field_image" for the image field name in Media. Before applying this patch, I could generate an image, but after saving it was lost entirely, and the media item had no thumbnail or anything.

After applying this patch, it works as expected -- after generating the image, it's properly attached to the media item.

Nice catch, thanks @kevinfunk. Merged.

Automatically closed - issue fixed for 2 weeks with no activity.