Thanks @muhammedyuzuak
I have found the solution that I was looking for. The API call needs to return the attachment in this format:
UEsDBAoAAAAAAHZXRlMb8/ygFgAAABYAAAGVzdC50eHR0aGlzIGlzIHNvbWUgdGVzdCB0ZXh0UEsBAj8ACgAAAAAAdldGUxvz/KAWAAAAFgAAAAgAJAAAAAAAAAAgAAAAAAAAAHRlc3QudHh0CgAgAAAAAAABABgAv57iEEW61wG/nuIQRbrXAcZ34hBFutcBUEsFBgAAAAABAAEAWgAAADwAAAAAAA==
I need to put this result into a string and decode it based on Base64, explained below. This helps me convert and save it to a .xlsx file.
Convert Base64 string to Image - Help / Studio - UiPath Community Forum