[Proposal] Use current mask upload UI for effective region mask input (Attn Mask)

[huchenlei]

SD Forge has the mask upload UI for effective region mask. It can be confusing if we have the UI doing something else here. Since we recently have effective region mask support implemeneted in #2785 and #2818. We should sync the mask upload behaviour with SD Forge.

BTW, I found the option can be a good compliment of the region control UI, as it gives users fine grained control on mask. Here is a potential usage example: https://www.youtube.com/watch?v=CSEnlkXHoIA&t=47s&pp=ygULQWNhZGVtaWEgU0Q%3D

[huchenlei]

Related discussion #2793.