fix(Server): validate express.static.mime.types

fixes: #1724

Author: hiroppy

lib/Server.js

@@ -89,12 +89,12 @@ class Server {
     if (options.lazy && !options.filename) {
       throw new Error("'filename' option must be set in lazy mode.");
     }
-    
+
     // if the user enables http2, we can safely enable https
     if (options.http2 && !options.https) {
       options.https = true;
     }
-    
+
     updateCompiler(compiler, options);
 
     this.stats =
@@ -174,7 +174,14 @@ class Server {
 
     // ref: https://github.com/webpack/webpack-dev-server/issues/1575
     // remove this when send@^0.16.3
-    express.static.mime.types.wasm = 'application/wasm';
+    if (
+      express.static &&
+      express.static.mime &&
+      express.static.mime.types &&
+      !express.static.mime.types.wasm
+    ) {
+      express.static.mime.types.wasm = 'application/wasm';
+    }
 
     app.all('*', (req, res, next) => {
       if (this.checkHost(req.headers)) {