How do I make transparent parts permeable?

Is it possible to make transparent parts permeable?

I drew tilemap

and designed some concrete floor for my platformer:

Unfortunately, the character feels support from the entire tilenot only from the non-transparent part.

I know I can draw separate invisible platforms. But is it possible to use tilemap directly?

I have youtube video, demonstrating a behavior, but the forum ate the link.

From the Manual on TileMap

A custom collision polygon can be set for individual tiles by clicking the Edit tile collision polygon button in the Tilemap bar, or just double-clicking the tile in the Tilemap bar.

No way to use transparency data automatically? Should I assume characters move on top of tiles all the time?