File upload in GCP and get expiry viewing link

LogoController.kt

package com.company.service.controllers

import com.google.cloud.storage.BlobInfo
import com.google.cloud.storage.Storage
import com.google.cloud.storage.StorageOptions
import kotlinx.coroutines.reactive.awaitFirstOrNull
import org.springframework.http.MediaType
import org.springframework.http.codec.multipart.FilePart
import org.springframework.web.bind.annotation.PostMapping
import org.springframework.web.bind.annotation.RequestPart
import org.springframework.web.bind.annotation.RestController
import java.io.DataInputStream
import java.io.File
import java.io.FileInputStream
import java.util.*
import java.util.concurrent.TimeUnit
import kotlin.random.Random


@RestController
class LogoController{

    private fun checkFileExtension(fileName: String?): Boolean {
        if (fileName != null && fileName.isNotEmpty() && fileName.contains(".")) {
            val allowedExt = arrayOf(".png")
            for (ext in allowedExt) {
                if (fileName.endsWith(ext)) {
                    return true
                }
            }
            throw Exception("file must be an image")
        }
        return false
    }

    @PostMapping("/imageUpload", consumes = [MediaType.MULTIPART_FORM_DATA_VALUE])
    // Move the logic to service
    suspend fun upload(@RequestPart("file") fileStream: FilePart): String {

        val storage = StorageOptions.getDefaultInstance().service

        checkFileExtension(fileStream.filename())

        val r = UUID.randomUUID().toString()

        val uploadFileName = "student/$r.png"

        val f = File("/tmp/$r.png")

        fileStream.transferTo(f).awaitFirstOrNull()

        val bytes = f.readBytes()


        val bucketName = "profile-forgetcode"

        val createdBlob = storage.create(
                BlobInfo
                        .newBuilder(bucketName, uploadFileName)
                        .setContentType("image/png")
                        .build(), bytes
        )

        val url = storage.signUrl(createdBlob, 15, TimeUnit.MINUTES, Storage.SignUrlOption.withV4Signature())

        f.delete()

        return url.toString()
    }
}