Function copy_to_multipart 

pub async fn copy_to_multipart<A: ResourceAccess, U: Upstream, O: DeserializeOwned + DeclaredFields>(
    access: &A,
    source_scope: &A::Scope,
    source: &ResourceReference,
    upstream: &U,
    target: &U::Target,
    template: WireRequest<()>,
    boundary: String,
    fields: Vec<MultipartPart>,
    filename: Option<String>,
    limits: CodecLimits,
) -> Result<JsonInvocation<O>, UploadError>

Downloads a source resource and sends an actual selected Files upload. This does not use ResourceAccess::publish or treat publication as file upload. Source is fully bounded/preflighted before the target side effect begins.