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The callback that is invoked after an asset is added to UploadAssetSourceType. When selecting an asset to upload, a default AssetDefinition object is constructed based on the selected asset and the callback is invoked. You can either leave the asset definition unmodified and do nothing (that's what the default implementation of the callback does), or adjust the properties of the object, or maybe even upload the asset file to your server and adjust the uri property of the asset. Note that the "upload" coroutine job will survive configuration changes and will be cancelled only if the editor is closed or the process is killed when in the background.