Why use multipart to upload an image?

Question

I am currently developing an app in Kotlin that uses the FACE API of Azure. To identify faces on images I need to send the image to the server. I use Retrofit 2.7.0 for the REST requests. Whenever I google about sending an image with retrofit, I come across the @Multipart annotation. For example here or here. None of the questions state why they do it. I found that apparently Multipart is the standard to send files via http.

However I do not seem to need it for my request. The simple approach seems to work just fine. Seeing as everyone else seems to use multipart, I am probably missing something. So my question is, why would I need to use Multipart over the simple approach?

I currently use this approach:

interface FaceAPI {
    @Headers(value = ["$CONTENT_TYPE_HEADER: $CONTENT_TYPE_OCTET_STREAM"])
    @POST("face/v1.0/detect")
    suspend fun detectFace(
        @Query("recognitionModel") recognitionModel: String = RECOGNITION_MODEL_2,
        @Query("detectionModel") detectionModel: String = DETECTION_MODEL_2,
        @Query("returnRecognitionModel") returnRecognitionModel: Boolean = false,
        @Query("returnFaceId") returnFaceId: Boolean = true,
        @Query("returnFaceLandmarks") returnFaceLandmarks: Boolean = false,
        @Header(HEADER_SUBSCRIPTION_KEY) subscriptionKey: String = SubscriptionKeyProvider.getSubscriptionKey(),
        @Body image: RequestBody
    ): Array<DetectResponse>
}

And then I call it like this:

suspend fun detectFaces(image: InputStream): Array<DetectResponse> {
    return withContext(Dispatchers.IO) {
        val bytes = image.readAllBytes()
        val body = bytes.toRequestBody(CONTENT_TYPE_OCTET_STREAM.toMediaTypeOrNull(), 0, bytes.size)
        val faceApi = ApiFactory.createFaceAPI()
        faceApi.detectFace(image = body)
    }
}

This code works for images up to the 6 MB that Azure supports.

Quentin · Accepted Answer · 2019-12-23T02:23:15.387

If you:

aren't generating the request by submitting an HTML form (which has native support for multipart but not for raw files)
don't need to convey multiple pieces of data (e.g. other form fields)

… then there is no need to use multipart.

Given the prevalence of it (due to the history from HTML form support) there are more server side data handling libraries which can handle it then there are for raw files so it may be easier to use multipart with some server side environments.

Why use multipart to upload an image?

1 Answers1