Move the opening of PDF file attachments into the DownloadManager-implementations

Note how the `PDFAttachmentViewer` handles PDF file attachments specially, by opening them in a new window/tab, rather than forcing them to be downloaded. This is done to improve the overall UX, since browsers in general are able to handle PDF files internally.
However, for file *annotations* we're currently not attempting to do the same thing and are instead just downloading them directly. In order to unify the behaviour, without having to duplicate a lot of code, the opening of PDF file attachments is thus moved into a new `DownloadManager.openOrDownloadData` method.

web/ui_utils.js

@@ -69,6 +69,9 @@ const SpreadMode = {
 // Used by `PDFViewerApplication`, and by the API unit-tests.
 const AutoPrintRegExp = /\bprint\s*\(/;
 
+// Used by the (various) `DownloadManager`-implementations.
+const PdfFileRegExp = /\.pdf$/i;
+
 // Replaces {{arguments}} with their values.
 function formatL10nValue(text, args) {
   if (!args) {
@@ -1059,6 +1062,7 @@ export {
   normalizeWheelEventDirection,
   NullL10n,
   parseQueryString,
+  PdfFileRegExp,
   PresentationModeState,
   ProgressBar,
   RendererType,