Introduced a simple way to work with files inside frontend

The usecase of this code is something like this: let image that user is making a
protonmail or any another application where content inside frontend (=browser
storage) some secret user's data that shouldn't be exposed to backend. Secret
key for example or anything like that.

This code allows to developer to save any `Array[Byte]` from frontend as simple
call `FileService.save()` and load any uploaded file to `Array[Byte]` as
`FileService.load()`.

Unfortunately scalatags doesn't support `download` attribute and I need to make
it by hand. I've opened a PR: com-lihaoyi/scalatags#212 to
introduce it, but it might be a while until it is included to release.

Author: catap

core/.js/src/main/scala/io/udash/utils/FileService.scala

@@ -0,0 +1,45 @@
+package io.udash.utils
+
+import java.io.IOException
+
+import org.scalajs.dom._
+import scalatags.JsDom.all._
+
+import scala.scalajs.js
+import js.JSConverters._
+import js.typedarray._
+
+import scala.concurrent._
+import scala.util.Try
+
+object FileService {
+  def save(filename: String, bytes: Array[Byte], mimeType: String = "application/octet-stream"): Unit = {
+    val jsBytes = Array[js.Any](bytes.toTypedArray).toJSArray
+    val blob = new Blob(jsBytes, BlobPropertyBag(mimeType))
+    val url = URL.createObjectURL(blob)
+    val download = attr("download")
+
+    val anchor = a(href := url, download := filename, style := "display: none")(filename).render
+    document.body.appendChild(anchor)
+    anchor.click()
+    document.body.removeChild(anchor)
+    URL.revokeObjectURL(url)
+  }
+
+  def load(file: File): Future[Array[Byte]] = {
+    val fileReader = new FileReader()
+    val promise = Promise[Array[Byte]]()
+
+    fileReader.onerror = (e: Event) =>
+      promise.failure(new IOException(e.toString))
+
+    fileReader.onload = (_: UIEvent) =>
+      promise.complete(Try(
+        new Int8Array(fileReader.result.asInstanceOf[ArrayBuffer]).toArray
+      ))
+
+    fileReader.readAsArrayBuffer(file)
+
+    promise.future
+  }
+}