R How to post three-dimensional array to plumber API?

Question

I have trained a neural network on an image classification task and now try to build an API with plumber to pass data to the model and get back the predictions.

But I am stuck on how to pass a three-dimensional array (the image) to the API. I tried different httr::POST calls and conversions from/to JSON, but could not get it working. The postBody of the request is a list of numbers and not an array anymore.

access_api.R

# Pass array to API
X <- array(rnorm(300 * 200 * 3), dim = c(300, 200, 3))
res <- httr::POST("http://127.0.0.1:6060/predict",
                  body = list(X),
                  encode = "json")

predict.R

#* @param X array
#* @post /predict
predict <- function(req) {

  req$postBody

  # Train model ...
}

plumber.R

library(plumber)
r <- plumb("predict.R")
r$run(port = 6060, host = "0.0.0.0")

I also tried converting the array to JSON, but then the structure of the array is lost and need to be restored:

A <- RJSONIO::toJSON(X)
B <- RJSONIO::fromJSON(A)

str(B)

D <- lapply(B, function(x) {
  Reduce(rbind, x)
})

E <- array(unlist(D), dim = dim(X))
str(E)