fix error loading .xnb files from the local mod folder since SMAPI 3.0

docs/release-notes.md

@@ -12,6 +12,9 @@
   * Added error message if you manually install the wrong SMAPI bitness (e.g. 32-bit SMAPI with 64-bit game).
   * Fixed intermittent error if a mod fetches mod-provided APIs asynchronously.
 
+* For mod authors:
+  * Fixed error loading `.xnb` files from the local mod folder since SMAPI 3.0.
+
 ## 3.11.0
 Released 09 July 2021 for Stardew Valley 1.5.4 or later. See [release highlights](https://www.patreon.com/posts/53514295).
 

src/SMAPI/Framework/ContentManagers/ModContentManager.cs

@@ -77,6 +77,8 @@ namespace StardewModdingAPI.Framework.ContentManagers
         /// <inheritdoc />
         public override T Load<T>(string assetName, LanguageCode language, bool useCache)
         {
+            // normalize key
+            bool isXnbFile = Path.GetExtension(assetName).ToLower() == ".xnb";
             assetName = this.AssertAndNormalizeAssetName(assetName);
 
             // disable caching
@@ -108,7 +110,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
             try
             {
                 // get file
-                FileInfo file = this.GetModFile(assetName);
+                FileInfo file = this.GetModFile(isXnbFile ? $"{assetName}.xnb" : assetName); // .xnb extension is stripped from asset names passed to the content manager
                 if (!file.Exists)
                     throw GetContentError("the specified path doesn't exist.");