Open PDF attachments in the viewer instead of download

If users want to download, they can quickly click on the Download button
in the newly opened viewer.

The blobUrl logic for Firefox relies on `disableCreateObjectURL` is
never false in Firefox. If the assumption is invalid, then PDF
attachments at the attachment view will not correctly be displayed,
because a data-URL will be generated and `?<filename>` is treated as
part of the data:-URL.

web/pdf_attachment_viewer.js

@@ -86,6 +86,38 @@ var PDFAttachmentViewer = (function PDFAttachmentViewerClosure() {
       this._renderedCapability.resolve();
     },
 
+    /**
+     * @private
+     */
+    _bindPdfLink:
+        function PDFAttachmentViewer_bindPdfLink(button, content, filename) {
+      var blobUrl;
+      button.onclick = function() {
+        if (!blobUrl) {
+          blobUrl = pdfjsLib.createObjectURL(
+            content, 'application/pdf', pdfjsLib.PDFJS.disableCreateObjectURL);
+        }
+        var viewerUrl;
+        if (typeof PDFJSDev === 'undefined' || PDFJSDev.test('GENERIC')) {
+          // The current URL is the viewer, let's use it and append the file.
+          viewerUrl = '?file=' + encodeURIComponent(blobUrl + '#' + filename);
+        } else if (PDFJSDev.test('CHROME')) {
+          // In the Chrome extension, the URL is rewritten using the history API
+          // in viewer.js, so an absolute URL must be generated.
+          // eslint-disable-next-line no-undef
+          viewerUrl = chrome.runtime.getURL('/content/web/viewer.html') +
+            '?file=' + encodeURIComponent(blobUrl + '#' + filename);
+        } else {
+          // Let Firefox's content handler catch the URL and display the PDF.
+          // In Firefox PDFJS.disableCreateObjectURL is always false, so
+          // blobUrl is always a blob:-URL and never a data:-URL.
+          viewerUrl = blobUrl + '?' + encodeURIComponent(filename);
+        }
+        window.open(viewerUrl);
+        return false;
+      };
+    },
+
     /**
      * @private
      */
@@ -124,11 +156,18 @@ var PDFAttachmentViewer = (function PDFAttachmentViewerClosure() {
       for (var i = 0; i < attachmentsCount; i++) {
         var item = attachments[names[i]];
         var filename = pdfjsLib.getFilenameFromUrl(item.filename);
+        filename = pdfjsLib.removeNullCharacters(filename);
+
         var div = document.createElement('div');
         div.className = 'attachmentsItem';
         var button = document.createElement('button');
+        button.textContent = filename;
+        if (/\.pdf$/i.test(filename)) {
+          this._bindPdfLink(button, item.content, filename);
+        } else {
           this._bindLink(button, item.content, filename);
-        button.textContent = pdfjsLib.removeNullCharacters(filename);
+        }
+
         div.appendChild(button);
         this.container.appendChild(div);
       }