Is it possible to get a callback before Unity begins importing?

I would need to “pre-process” files before Unity starts importing them (and potentially cancel or defer the Unity import). Is there any way to get a callback before the import begins?

Yes, you can use an AssetPostProcessor. Though i’m not sure if it’s possible to prevent / cancel the importing during the preprocess callback. In general you can’t decide which assets are imported and which aren’t. This soley depends on the location of the file and the file type / extension.