lightbox: Download images with correct names and MIME types.

src/lightbox/LightboxActionSheet.js

@@ -42,8 +42,9 @@ const tryToDownloadImage = async ({ src, auth }: DownloadImageType) => {
     return;
   }
 
+  const fileName = src.split('/').pop();
   try {
-    await downloadImage(tempUrl, auth);
+    await downloadImage(tempUrl, fileName, auth);
     showToast('Download complete');
   } catch (error) {
     showToast(error.message);

src/lightbox/downloadImage.js

@@ -3,6 +3,7 @@ import { CameraRoll, Platform, PermissionsAndroid } from 'react-native';
 import RNFetchBlob from 'rn-fetch-blob';
 
 import type { Auth } from '../api/transportTypes';
+import { getMimeTypeFromFileExtension } from '../utils/url';
 
 /**
  * Request permission WRITE_EXTERNAL_STORAGE, or throw if can't get it.
@@ -36,21 +37,24 @@ const androidEnsureStoragePermission = async (): Promise<void> => {
  *
  * @param url A URL to the image.  Should be a valid temporary URL generated
  *     using `getTemporaryFileUrl`.
+ * @param fileName Name of the file to be downloaded. Should include the
+ *     extension.
  * @param auth Authentication info for the current user.
  */
-export default async (url: string, auth: Auth): Promise<mixed> => {
+export default async (url: string, fileName: string, auth: Auth): Promise<mixed> => {
   if (Platform.OS === 'ios') {
     return CameraRoll.saveToCameraRoll(url);
   }
 
   // Platform.OS === 'android'
+  const mime = getMimeTypeFromFileExtension(fileName.split('.').pop());
   await androidEnsureStoragePermission();
   return RNFetchBlob.config({
     addAndroidDownloads: {
-      path: `${RNFetchBlob.fs.dirs.DownloadDir}/${url.split('/').pop()}`,
+      path: `${RNFetchBlob.fs.dirs.DownloadDir}/${fileName}`,
       useDownloadManager: true,
-      mime: 'text/plain', // Android DownloadManager fails if the url is missing a file extension
-      title: url.split('/').pop(),
+      mime, // Android DownloadManager fails if the url is missing a file extension
+      title: fileName,
       notification: true,
     },
   }).fetch('GET', url);

src/lightbox/shareImage.js

@@ -18,12 +18,13 @@ export default async (url: string, auth: Auth) => {
     return;
   }
 
+  const fileName = url.split('/').pop();
   if (Platform.OS === 'android') {
-    const res: $FlowFixMe = await downloadImage(tempUrl, auth);
+    const res: $FlowFixMe = await downloadImage(tempUrl, fileName, auth);
     await ShareImageAndroid.shareImage(res.path());
   } else {
     try {
-      const uri = await downloadImage(tempUrl, auth);
+      const uri = await downloadImage(tempUrl, fileName, auth);
       try {
         await Share.share({ url: uri, message: url });
       } catch (error) {