After some hours of search and destroy tactical operations,

I found it 😂 spotted

I had to place a file called

srs/main/resources/META-INF/native-image/reflect-config.json

with these contents:

[
  {
    "name": "org.springframework.http.codec.multipart.FilePart",
    "allDeclaredConstructors": true,
    "allDeclaredMethods": true,
    "allDeclaredFields": true
  },
  {
    "name": "org.springframework.http.codec.multipart.FilePart[]",
    "unsafeAllocated": true
  }
]

And hooray, it finally worked