adjust ModContentManager.HandleUnknownFileTypes to let mods patch it

docs/release-notes.md

@@ -18,6 +18,7 @@
 * For mod authors:
   * Added `IsActiveForScreen()` method to `PerScreen<T>`.
   * Updated to [FluentHttpClient](https://github.com/Pathoschild/FluentHttpClient#readme) 4.3.0 (see [changes](https://github.com/Pathoschild/FluentHttpClient/blob/develop/RELEASE-NOTES.md#430)).
+  * Adjusted `ModContentManager.HandleUnknownFileType` to let mods patch it.
   * Fixed `Context.IsWorldReady` being editable by mods.
 
 ## 3.18.2

src/SMAPI/Framework/ContentManagers/ModContentManager.cs

@@ -130,7 +130,7 @@ namespace StardewModdingAPI.Framework.ContentManagers
                     ".png" => this.LoadImageFile<T>(assetName, file),
                     ".tbin" or ".tmx" => this.LoadMapFile<T>(assetName, file),
                     ".xnb" => this.LoadXnbFile<T>(assetName),
-                    _ => this.HandleUnknownFileType<T>(assetName, file)
+                    _ => (T)this.HandleUnknownFileType(assetName, file, typeof(T))
                 };
             }
             catch (Exception ex)
@@ -323,13 +323,15 @@ namespace StardewModdingAPI.Framework.ContentManagers
         }
 
         /// <summary>Handle a request to load a file type that isn't supported by SMAPI.</summary>
-        /// <typeparam name="T">The expected file type.</typeparam>
         /// <param name="assetName">The asset name relative to the loader root directory.</param>
         /// <param name="file">The file to load.</param>
-        private T HandleUnknownFileType<T>(IAssetName assetName, FileInfo file)
+        /// <param name="assetType">The expected file type.</param>
+        private object HandleUnknownFileType(IAssetName assetName, FileInfo file, Type assetType)
         {
             this.ThrowLoadError(assetName, ContentLoadErrorType.InvalidName, $"unknown file extension '{file.Extension}'; must be one of '.fnt', '.json', '.png', '.tbin', '.tmx', or '.xnb'.");
-            return default;
+            return assetType.IsValueType
+                ? Activator.CreateInstance(assetType)
+                : null;
         }
 
         /// <summary>Assert that the asset type is compatible with one of the allowed types.</summary>