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 awsCredential = BasicAWSCredentials(
                credential.credentials.accessKeyId,
                credential.credentials.secretAccessKey)

            val s3Client = AmazonS3Client(awsCredential, 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.key,
                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(
                            "${credential.bucket}/${session.user!!.uid}/files" +
                                "${key}.pdf")
                    }
                }

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

    }