diff --git a/src/DiffEngine.Tests/ExtensionCaseTests.cs b/src/DiffEngine.Tests/ExtensionCaseTests.cs
new file mode 100644
index 00000000..38ed8808
--- /dev/null
+++ b/src/DiffEngine.Tests/ExtensionCaseTests.cs
@@ -0,0 +1,39 @@
+/// <summary>
+/// Extensions are matched the way file systems produce them.
+/// <para>
+/// ExtensionLookup, PathLookup and BinaryExtensions all compared ordinally while every
+/// registration is lowercase, so .PNG, .JPG and .Docx - which is exactly what Windows and macOS
+/// hand back - matched nothing. The tool resolved for foo.png and not for foo.PNG.
+/// Viewer/ImageExtensions already uses OrdinalIgnoreCase for the same kind of lookup.
+/// </para>
+/// </summary>
+[NotInParallel]
+public class ExtensionCaseTests
+{
+    [Test]
+    public async Task ExtensionsResolveWhateverTheirCasing()
+    {
+        var extension = $".zz{Guid.NewGuid():N}";
+        var tool = DiffTools.AddTool(
+            name: $"CaseProbe{Guid.NewGuid():N}",
+            autoRefresh: false,
+            isMdi: false,
+            supportsText: false,
+            requiresTarget: true,
+            useShellExecute: false,
+            launchArguments: new(
+                Left: (temp, target) => $"\"{temp}\" \"{target}\"",
+                Right: (temp, target) => $"\"{target}\" \"{temp}\""),
+            exePath: Environment.ProcessPath!,
+            binaryExtensions: [extension]);
+
+        await Assert.That(tool).IsNotNull();
+
+        await Assert.That(DiffTools.TryFindByExtension(extension, out _)).IsTrue();
+        await Assert.That(DiffTools.TryFindByExtension(extension.ToUpperInvariant(), out _)).IsTrue();
+
+        await Assert.That(DiffTools.TryFindForInputFilePath($"file{extension.ToUpperInvariant()}", out _)).IsTrue();
+
+        await Assert.That(tool!.BinaryExtensions.Contains(extension.ToUpperInvariant())).IsTrue();
+    }
+}
diff --git a/src/DiffEngine/DiffTools.cs b/src/DiffEngine/DiffTools.cs
index 103cbf35..aeb3fb06 100644
--- a/src/DiffEngine/DiffTools.cs
+++ b/src/DiffEngine/DiffTools.cs
@@ -2,8 +2,12 @@
 
 public static partial class DiffTools
 {
-    static Dictionary<string, ResolvedTool> ExtensionLookup = [];
-    static Dictionary<string, ResolvedTool> PathLookup = [];
+    // Case insensitive, because the keys are file extensions and executable paths and the file
+    // systems that produce them are. An ordinal lookup meant .PNG, .JPG and .Docx - which is what
+    // Windows and macOS hand back - matched none of the lowercase registrations, so the tool
+    // resolved for foo.png and not for foo.PNG. Viewer/ImageExtensions already does this
+    static Dictionary<string, ResolvedTool> ExtensionLookup = new(StringComparer.OrdinalIgnoreCase);
+    static Dictionary<string, ResolvedTool> PathLookup = new(StringComparer.OrdinalIgnoreCase);
     static Dictionary<DiffTool, ResolvedTool> ToolLookup = [];
     static ResolvedTool? firstTextTool;
     static List<ResolvedTool> resolved = [];
diff --git a/src/DiffEngine/ResolvedTool.cs b/src/DiffEngine/ResolvedTool.cs
index d94e547b..24f1f4de 100644
--- a/src/DiffEngine/ResolvedTool.cs
+++ b/src/DiffEngine/ResolvedTool.cs
@@ -62,7 +62,9 @@ Extensions must begin with a period.
             }
         }
 
-        BinaryExtensions = binaryExtensions.ToFrozenSet();
+        // Case insensitive for the same reason ExtensionLookup is: a binary extension is compared
+        // against whatever casing the file system produced
+        BinaryExtensions = binaryExtensions.ToFrozenSet(StringComparer.OrdinalIgnoreCase);
 
         RequiresTarget = requiresTarget;
         SupportsText = supportsText;