Upload files using OkHttp multi-part request

FileUploader.kt

fun uploadFile(
      url: String,
      headers: Map<String, String>,
      files: Map<String, Attachment>,
      formDataParts: Map<String, Any>
  ): Request {
      val body =
          MultipartBody.Builder()
              .setType(MultipartBody.FORM)
              .apply {
                  formDataParts.forEach { (key, value) -> addFormDataPart(key, value.toString()) }
                  files.forEach { (field, attachment) ->
                      addFormDataPart(
                          field,
                          attachment.file.name,
                          attachment.file.asRequestBody(attachment.mediaType.toMediaTypeOrNull())
                      )
                  }
              }
              .build()

      return Request.Builder()
          .url(url)
          .addHeader("Content-Type", "multipart/form-data")
          .apply { headers.forEach { (name, value) -> addHeader(name, value) } }
          .post(body)
          .build()
  }

class Attachment(
    val file: File,
    val mediaType: String
)