Reproduction steps:
1. Open the user-attached project
2. Observe the Console

Expected result: No unclearable errors, the MultiSpriteSVGImporter custom importer is registered and used for .multi.svg files
Actual result: Importer registration fails with a wrong/confusing error

Reproduced in: 2020.3.39f1, 2021.3.9f1, 2022.1.0a15
Not reproduced in: 2022.1.0a16, 2022.1.16f1, 2022.2.0b7, 2023.1.0a9

Reproduced using: Windows 11, macOS Monterey 12.0.1

Full error:
The scripted importer 'MultiSpriteSVGImporter' attempted to register file type '.multi.svg', which is handled by a native Unity importer. Registration rejected.
UnityEditor.AssetImporters.ScriptedImporter:RegisterScriptedImporters ()