Custom generator of previews. Custom code to generate previews. Please check [code]file_dialog/thumbnail_size[/code] in [EditorSettings] to find out the right size to do previews at. If this function returns [code]true[/code], the generator will call [method _generate] or [method _generate_from_path] for small previews as well. By default, it returns [code]false[/code]. Generate a preview from a given resource with the specified size. This must always be implemented. Returning [code]null[/code] is an OK way to fail and let another generator take care. Care must be taken because this function is always called from a thread (not the main thread). [param metadata] dictionary can be modified to store file-specific metadata that can be used in [method EditorResourceTooltipPlugin._make_tooltip_for_path] (like image size, sample length etc.). Generate a preview directly from a path with the specified size. Implementing this is optional, as default code will load and call [method _generate]. Returning [code]null[/code] is an OK way to fail and let another generator take care. Care must be taken because this function is always called from a thread (not the main thread). [param metadata] dictionary can be modified to store file-specific metadata that can be used in [method EditorResourceTooltipPlugin._make_tooltip_for_path] (like image size, sample length etc.). If this function returns [code]true[/code], the generator will automatically generate the small previews from the normal preview texture generated by the methods [method _generate] or [method _generate_from_path]. By default, it returns [code]false[/code]. Returns [code]true[/code] if your generator supports the resource of type [param type].