Add `Attachment` type

Defines various functions on attachments, e.g. normalization (auto-orient JPEGs, etc.)

js/modules/types/attachment.js

@@ -0,0 +1,68 @@
+/* jshint ignore:start */
+
+const {autoOrientImage} = require('../auto_orient_image');
+
+// // Fields
+// {
+//   "id": "7494241695219027913",
+//   "contentType": "image/jpeg",
+//   "key": {},
+//   "size": 352727,
+//   "thumbnail": null,
+//   "digest": {},
+//   "fileName": "Landscape_6.jpg",
+//   "flags": null,
+//   "data": {}
+// }
+
+const isJPEG = mimeType =>
+  mimeType === 'image/jpeg' || mimeType === 'image/jpg';
+
+// Data type conversion
+const blobToArrayBuffer = blob => {
+  return new Promise((resolve, reject) => {
+    const fileReader = new FileReader();
+
+    fileReader.onload = event =>
+        resolve(event.target.result);
+
+    fileReader.readAsArrayBuffer(blob);
+  });
+};
+
+const arrayBufferToBlob = (arrayBuffer, mimeType) =>
+  new Blob([arrayBuffer], {type: mimeType});
+
+// Processing steps:
+const withLastNormalizedDate = attachment => Object.assign(
+  {},
+  attachment,
+  {lastNormalized: new Date().toISOString()},
+);
+
+const autoOrientJPEGs = async attachment => {
+  if (!isJPEG(attachment.contentType)) {
+    return attachment;
+  }
+
+  const dataBlob = arrayBufferToBlob(attachment.data, attachment.contentType);
+  const newDataBlob = dataURLtoBlob(await autoOrientImage(dataBlob));
+  const newDataArrayBuffer = await blobToArrayBuffer(newDataBlob);
+  const newAttachment = Object.assign({}, attachment, {
+    data: newDataArrayBuffer,
+    size: newDataArrayBuffer.byteLength,
+    lastNormalized: new Date().toISOString(),
+  });
+
+  // `digest` is no longer valid for auto-oriented image data, so we discard it:
+  delete newAttachment.digest;
+
+  return newAttachment;
+};
+
+// Public API
+
+// Attachment -> Promise<Attachment>
+exports.normalize = async attachment =>
+  withLastNormalizedDate(await autoOrientJPEGs(attachment));
+