Drag a non standard unity asset on a custom editor slot

@hexdump:
If that doesn't answer your question you might be more specific about what you want to do. You said you "want to handle a psd file in unity". When you simply drag a psd file into your project it is automatically imported as Texture2D. If you need some kind of special handling you should be more specific.

The Unity editor is designed to work with assets which are located in the assets folder. The ObjectField can only handle classes which are derived from UnityEngine.Object.

You said you want a GUI window where the user should drag and drop the file. Where does the file come from? If you talk about files which are no assets, so they are located outside of your project's assets folder, that's not possible. For that feature you would need to activate drag and drop support of the OS to make it send "WM_DROPFILES" messages to your window. Since you don't have access to the editor window message pump, even when you allow drag and drop events you can't receive the data.

The only way around that would be to write a native code plugin which shows a native window which you control yourself.

Anyways, we can't suggest anything without knowing what you want to do.