1. Create a new project in 3D
2. Import any 2D Sprite
3. Try to drag the sprite into scene view

User is unable to drag the sprite into scene view. If user toggles 2D mode, they are still unable to drag sprite into scene view.
User should be able to drag a 2D sprite into scene view, regardless of 2D/3D mode.

Note: If user goes into Edit > Project Settings > Editor > Default Behavour Mode > change from 3D to 2D, they are now able to drag into scene view.

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Original bug description follows:

How to reproduce:
1. Open the attached project
2. Create a new scene or use existing one
3. Select sprite from project folder
4. Try dragging it to the scene view
Result: it is not allowed, one can drag it only to hierarchy

Reproducible: 5.6.0b2, 5.6.0b3
Not reproducible: 5.6.0b1