[piexwasm] Use MIME type in |request.js|

Use the mimeType property that was sent from piex_loader.js to prevent
lossy JPEG encoding of images that are of type BMP.

Bug: 1039141
Change-Id: I1ee989bb8c2627cd5fba282dd7bb0b7214b48f54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2060358
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742035}

ui/file_manager/image_loader/request.js

@@ -310,10 +310,8 @@ ImageRequest.prototype.downloadOriginal_ = function(onSuccess, onFailure) {
               this.request_.orientation = data.orientation;
               this.request_.colorSpace = data.colorSpace;
               this.ifd_ = data.ifd;
-              // TODO(1039141): if piex-wasm is updated to return more than
-              // just JPEG then it should indicate the image mimeType so we
-              // can set this.contentType_.
-              const blob = new Blob([data.thumbnail], {type: 'image/jpeg'});
+              this.contentType_ = data.mimeType;
+              const blob = new Blob([data.thumbnail], {type: data.mimeType});
               this.image_.src = URL.createObjectURL(blob);
             }.bind(this),
             function() {