feat(PDF):Add PDF viewer (#2730)

Display DICOM encapsulated PDF documents in a PDF viewer.
Addressed the remaining PR comments, and am merging this.

platform/viewer/.webpack/webpack.pwa.js

@@ -24,12 +24,16 @@ const ENTRY_TARGET = process.env.ENTRY_TARGET || `${SRC_DIR}/index.js`;
 const Dotenv = require('dotenv-webpack');
 
 const setHeaders = (res, path) => {
-  res.setHeader('Content-Type', 'text/plain')
   if (path.indexOf('.gz') !== -1) {
     res.setHeader('Content-Encoding', 'gzip')
   } else if (path.indexOf('.br') !== -1) {
     res.setHeader('Content-Encoding', 'br')
   }
+  if (path.indexOf('.pdf') !== -1) {
+    res.setHeader('Content-Type', 'application/pdf');
+  } else {
+    res.setHeader('Content-Type', 'application/json')
+  }
 }
 
 module.exports = (env, argv) => {