R1:3367fe0017b1

Drag & Drop Task Cover Image: also attach

Summary:
After this change, the cover image is finally always visible to others (to people who can see the Task)
instead of being visible only to yourself.

Example conversation before this change:

- Anna: (uploads a nice Cover Image on the Task, using drag & drop from the Workboard)
- Bob: Thanks! But I cannot see that image. Please change visibility.
- Anna: Ouch! How?
- Bob: I don't know. Try Files > your File > Edit > Set Visibility > Something that includes…