Is there any functional difference between

@PostMapping
suspend fun uploadFile(@RequestParam file: Mono<FilePart>)

and

@PostMapping
suspend fun uploadFile(@RequestParam file: FilePart)

other than maybe allow the former to not pass any value?

Does your first example actually work? I was under the impression that Spring 'translated' Mono/Flux to suspend variants behind the scenes. e.g.

@PostMapping
fun uploadFile(@RequestParam file: Mono<FilePart>)

to

@PostMapping
suspend fun uploadFile(@RequestParam file: FilePart)