private fun uploadFileToS3Bucket(filePath: String, credential: AwsCredentialResponse): Maybe<String> {

        // from blog: http://blog.mallow-tech.com/2019/01/how-to-upload-files-from-an-android-app-to-aws-s3/
        return Maybe.create { emitter ->

            TransferNetworkLossHandler.getInstance(context)

            val simpleDateFormat = SimpleDateFormat(DateUtils.FORMAT_UTC, Locale.ENGLISH)

            val expirationDate = try {
                simpleDateFormat.parse(credential.credentials.expiration)
            } catch (e: Exception) {
                val calendar = Calendar.getInstance()
                calendar.add(Calendar.MINUTE, 10) // expiration time is 10 minutes from now
                Date(calendar.timeInMillis)
            }

            val s3Client: AmazonS3 =
                AmazonS3Client(
                    AwsCredential(
                        credential.credentials.accessKeyId,
                        credential.credentials.accessKeyId,
                        credential.credentials.sessionToken,
                        expirationDate
                    ),
                    getRegion(Regions.EU_WEST_1)
                )

            val file = File(filePath)

            val transferUtility =
                TransferUtility.builder().s3Client(s3Client).context(context).build()

            val transferObserver: TransferObserver = transferUtility.upload(
                credential.bucket,
                credential.keyPrefix,
                file,
                CannedAccessControlList.PublicRead
            )

            transferObserver.setTransferListener(object : TransferListener {

                override fun onStateChanged(id: Int, state: TransferState?) {

                    when (state) {

                        TransferState.IN_PROGRESS -> {

                        }
                        TransferState.COMPLETED -> {

                        }
                        TransferState.FAILED -> {

                        }
                        else -> {

                        }
                    }
                }

                override fun onProgressChanged(
                    id: Int,
                    bytesCurrent: Long,
                    bytesTotal: Long
                ) {
                    if (bytesCurrent == bytesTotal) {
                        emitter.onSuccess("${credential.bucket}/${credential.keyPrefix}/${file.name}")
                    }
                }

                override fun onError(id: Int, e: Exception) {
                    emitter.onError(e)
                }
            })
        }

    }