[Static Web Assets] Fix ContentTypeProvider to respect custom content-type mappings precedence (#50153)

#fixes #50152 by giving custom mappings precedence over built-in ones.

Author: Copilot

src/StaticWebAssetsSdk/Tasks/Data/ContentTypeProvider.cs

@@ -459,7 +459,7 @@ private bool TryGetMapping(StaticWebAssetGlobMatcher.MatchContext context, TaskL
         var match = _matcher.Match(context);
         if (match.IsMatch)
         {
-            if (_builtInMappings.TryGetValue(match.Pattern, out mapping) || _customMappings.TryGetValue(match.Pattern, out mapping))
+            if (_customMappings.TryGetValue(match.Pattern, out mapping) || _builtInMappings.TryGetValue(match.Pattern, out mapping))
             {
                 log.LogMessage(MessageImportance.Low, $"Matched {relativePath} to {mapping.MimeType} using pattern {match.Pattern}");
                 return true;

test/Microsoft.NET.Sdk.StaticWebAssets.Tests/StaticWebAssets/ContentTypeProviderTests.cs

@@ -114,6 +114,57 @@ public void GetContentType_ReturnsTextPlainForCompressedTextFiles(string path)
         Assert.Equal("text/plain", contentType.MimeType);
     }
 
+    [Fact]
+    public void GetContentType_CustomMappingOverridesBuiltInMapping()
+    {
+        // Arrange
+        var customMapping = new ContentTypeMapping("text/html", "no-store, must-revalidate, no-cache", "*.html", 2);
+        var provider = new ContentTypeProvider([customMapping]);
+
+        // Act
+        var contentType = provider.ResolveContentTypeMapping(CreateContext("index.html"), _log);
+
+        // Assert
+        Assert.Equal("text/html", contentType.MimeType);
+        Assert.Equal("no-store, must-revalidate, no-cache", contentType.Cache);
+        Assert.Equal("*.html", contentType.Pattern);
+        Assert.Equal(2, contentType.Priority);
+    }
+
+    [Fact]
+    public void GetContentType_CustomMappingOverridesBuiltInMappingForCompressedFiles()
+    {
+        // Arrange
+        var customMapping = new ContentTypeMapping("text/html", "no-store, must-revalidate, no-cache", "*.html", 2);
+        var provider = new ContentTypeProvider([customMapping]);
+
+        // Act
+        var contentType = provider.ResolveContentTypeMapping(CreateContext("index.html.gz"), _log);
+
+        // Assert
+        Assert.Equal("text/html", contentType.MimeType);
+        Assert.Equal("no-store, must-revalidate, no-cache", contentType.Cache);
+        Assert.Equal("*.html", contentType.Pattern);
+        Assert.Equal(2, contentType.Priority);
+    }
+
+    [Fact]
+    public void GetContentType_CustomJavaScriptMappingOverridesBuiltIn()
+    {
+        // Arrange
+        var customMapping = new ContentTypeMapping("text/javascript", "max-age=3600", "*.js", 3);
+        var provider = new ContentTypeProvider([customMapping]);
+
+        // Act
+        var contentType = provider.ResolveContentTypeMapping(CreateContext("app.js"), _log);
+
+        // Assert
+        Assert.Equal("text/javascript", contentType.MimeType);
+        Assert.Equal("max-age=3600", contentType.Cache);
+        Assert.Equal("*.js", contentType.Pattern);
+        Assert.Equal(3, contentType.Priority);
+    }
+
     private class TestTaskLoggingHelper : TaskLoggingHelper
     {
         public TestTaskLoggingHelper() : base(new TestTask())