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Currently it's not possible to drag n drop flac files onto the mapping editor, but you can load them through the import samples dialog. Could you add the ability to drag n drop them too?
@d-healey https://github.com/LightandSound/HISE-Stage/blob/d44b6d912390820c0b49e0dbb73e09c1d03ca83c/hi_tools/hi_standalone_components/SampleDisplayComponent.cpp#L392
register FLAC in the audio format manager and you're done :)