Refactors the upload handler to use the `services.IncomingFile` interface instead of concrete `*multipart.FileHeader` pointers. This decouples the core upload logic from the HTTP multipart implementation, allowing for more flexible file sources.
Changes include:
- Introducing `namedMultipartFile` to adapt multipart headers to the new interface.
- Updating `createOrAppendBox`, `checkUploadPolicy`, and `totalUploadBytes` to accept `IncomingFile`.
- Renaming service calls to `CreateBoxFromIncoming` and `AppendIncomingFiles`.