fix(Ads): Fix unnecessary request when using VAST (#7660)

If the URL does not have an extension, using mimeType allows us to avoid
having to make a HEAD type request

Author: avelad

docs/tutorials/ad_monetization.md

@@ -101,6 +101,7 @@ adManager.addCustomInterstitial({
   startTime: 10,
   endTime: null,
   uri: 'YOUR_URL',
+  mimeType: null,
   isSkippable: true,
   skipOffset: 10,
   canJump: false,
@@ -136,6 +137,7 @@ player.addEventListener('timelineregionadded', (e) => {
     startTime: event.startTime,
     endTime: event.endTime,
     uri: 'YOUR_URL',
+    mimeType: null,
     isSkippable: false,
     skipOffset: null,
     skipFor: null,
@@ -171,6 +173,7 @@ adManager.addCustomInterstitial({
   startTime: 10,
   endTime: null,
   uri: 'YOUR_URL',
+  mimeType: null,
   isSkippable: true,
   skipOffset: 10,
   canJump: false,

externs/shaka/ads.js

@@ -63,6 +63,7 @@ shaka.extern.AdCuePoint;
  *   startTime: number,
  *   endTime: ?number,
  *   uri: string,
+ *   mimeType: ?string,
  *   isSkippable: boolean,
  *   skipOffset: ?number,
  *   skipFor: ?number,
@@ -89,6 +90,8 @@ shaka.extern.AdCuePoint;
  * @property {string} uri
  *   The uri of the interstitial, can be any type that
  *   ShakaPlayer supports (either in MSE or src=)
+ * @property {?string} mimeType
+ *   The mimeType of the interstitial if known.
  * @property {boolean} isSkippable
  *   Indicate if the interstitial is skippable.
  * @property {?number} skipOffset

lib/ads/ad_utils.js

@@ -80,6 +80,7 @@ shaka.ads.Utils = class {
             startTime: startTime,
             endTime: null,
             uri: adUrl,
+            mimeType: media.attributes['type'] || null,
             isSkippable: skipOffset != null,
             skipOffset,
             skipFor: null,

lib/ads/interstitial_ad_manager.js

@@ -181,7 +181,10 @@ shaka.ads.InterstitialAdManager = class {
             if (difference > 0 && difference <= 10) {
               if (!this.preloadManagerInterstitials_.has(interstitial)) {
                 this.preloadManagerInterstitials_.set(
-                    interstitial, this.player_.preload(interstitial.uri));
+                    interstitial, this.player_.preload(
+                        interstitial.uri,
+                        /* startTime= */ null,
+                        interstitial.mimeType || undefined));
               }
             }
           }
@@ -333,6 +336,7 @@ shaka.ads.InterstitialAdManager = class {
       startTime: region.startTime,
       endTime: region.endTime,
       uri: alternativeMPDUri,
+      mimeType: null,
       isSkippable: false,
       skipOffset: null,
       skipFor: null,
@@ -416,7 +420,10 @@ shaka.ads.InterstitialAdManager = class {
       if (shouldPreload) {
         if (!this.preloadManagerInterstitials_.has(interstitial)) {
           this.preloadManagerInterstitials_.set(
-              interstitial, this.player_.preload(interstitial.uri));
+              interstitial, this.player_.preload(
+                  interstitial.uri,
+                  /* startTime= */ null,
+                  interstitial.mimeType || undefined));
         }
       }
     }
@@ -779,10 +786,16 @@ shaka.ads.InterstitialAdManager = class {
         if (preloadManager) {
           await this.player_.load(preloadManager);
         } else {
-          await this.player_.load(interstitial.uri);
+          await this.player_.load(
+              interstitial.uri,
+              /* startTime= */ null,
+              interstitial.mimeType || undefined);
         }
       } else {
-        await this.player_.load(interstitial.uri);
+        await this.player_.load(
+            interstitial.uri,
+            /* startTime= */ null,
+            interstitial.mimeType || undefined);
       }
       if (interstitial.playoutLimit) {
         if (playoutLimitTimer) {
@@ -912,6 +925,7 @@ shaka.ads.InterstitialAdManager = class {
         startTime,
         endTime,
         uri,
+        mimeType: null,
         isSkippable,
         skipOffset,
         skipFor,
@@ -964,6 +978,7 @@ shaka.ads.InterstitialAdManager = class {
               startTime,
               endTime,
               uri: asset['URI'],
+              mimeType: null,
               isSkippable,
               skipOffset,
               skipFor,