interface FileApi {
    @Multipart @POST("/upload0")
    fun upload0(@Part("title") title: RequestBody, @Part file: MultipartBody.Part): Call<Int>
...
}

fun upload0() {
    val title = "my file"
    val titlePart = RequestBody.create(MultipartBody.FORM, title)

    val file = File(uploadDir, "horse.ogg")
    val filebody = RequestBody.create(MediaType.parse("multipart/form-data"), file)
    val filepart = MultipartBody.Part.createFormData("file", file.name, filebody)

    val call = fileApi.upload0(titlePart, filepart)
    val response = call.execute().body()
}

server side :
@location("/upload0") class Upload0

val multipart = call.request.receive<MultiPartData>()
if (call.request.isMultipart()) {
    multipart.parts.forEach { part ->
        if (part is PartData.FormItem) {
            if (part.partName == "title") {
                title = part.value
            }
        } else if (part is PartData.FileItem) {
            val ext = File(part.originalFileName).extension
            val file = File(uploadDir, "${System.currentTimeMillis()}.$ext")
            part.streamProvider().use { instream ->
                file.outputStream().buffered().use { outstream ->
                    instream.copyTo(outstream)
                }
            }
            fileName = file.name
            fileType = part.contentType?.contentType
        }
        part.dispose()
    }
}