fix error when mod loads XNB mod file without extension

docs/release-notes.md

@@ -3,6 +3,7 @@
 # Release notes
 ## Upcoming release
 * For mod authors:
+  * Fixed error when loading a `.xnb` file through the old content API without the file extension.
   * Fixed asset propagation for player sprites not fully updating recolor masks in some cases.
 
 * For the web UI:

src/SMAPI/Framework/ModHelpers/ContentHelper.cs

@@ -132,7 +132,32 @@ namespace StardewModdingAPI.Framework.ModHelpers
                         return this.GameContentManager.LoadLocalized<T>(assetName, this.CurrentLocaleConstant, useCache: false);
 
                     case ContentSource.ModFolder:
-                        return this.ModContentManager.LoadExact<T>(assetName, useCache: false);
+                        try
+                        {
+                            return this.ModContentManager.LoadExact<T>(assetName, useCache: false);
+                        }
+                        catch (SContentLoadException ex) when (ex.ErrorType == ContentLoadErrorType.AssetDoesNotExist)
+                        {
+                            // legacy behavior: you can load a .xnb file without the file extension
+                            try
+                            {
+                                IAssetName newName = this.ContentCore.ParseAssetName(assetName.Name + ".xnb", allowLocales: false);
+                                if (this.ModContentManager.DoesAssetExist<T>(newName))
+                                {
+                                    T data = this.ModContentManager.LoadExact<T>(newName, useCache: false);
+                                    SCore.DeprecationManager.Warn(
+                                        this.Mod,
+                                        "loading XNB files from the mod folder without the .xnb file extension",
+                                        "3.14.0",
+                                        DeprecationLevel.Notice
+                                    );
+                                    return data;
+                                }
+                            }
+                            catch { /* legacy behavior failed, rethrow original error */ }
+
+                            throw;
+                        }
 
                     default:
                         throw new SContentLoadException(ContentLoadErrorType.Other, $"{this.Mod.DisplayName} failed loading content asset '{key}' from {source}: unknown content source '{source}'.");