So! I have it figured out... Kinda? So I figured that my choice was architecturally wrong - if there's not a clear cut technical solution, probably something to be improved on the architecture side of things.

What I've done is created a separate endpoint for uploading files, that returns the UUID of the file in the response. I will upload all files associated with that JSON object BEFORE sending the main body, attaching the UUIDs of the files in the JSON fields appropriate.

Not the approach I intended, but this keeps the code cleaner overall. Thanks!