Question

It would be useful to have the ability to load an image as a background, specifically in Perspective Filter. I really like this filter and use it a lot so is there a way to adjust the shadow to fit the perspective of say a floor or a wall?

Answer

Here's an idea that may help you with this problem. I presume you are running the Shadow filter on a layer with opacity, like a text layer. I also presume you have a background image of a floor or wall handy. Here is what you do: 

1. Load the background floor/wall image into Photoshop. 
2. Create your text layer above the background image. Duplicate this layer so you can come back to it later. 
3. Merge the text layer and the background image into one layer.
4. Load the text layer (you saved a copy of it in step 2) as a selection using the menu: Select > Load Selection.
5. Run the Shadow filter on the merged layer you created in step 3. You will now be able to see the text and the floor/wall all in the preview. Adjust the shadow to your liking and then apply it.