diff --git a/src/vs/workbench/api/node/extHost.api.impl.ts b/src/vs/workbench/api/node/extHost.api.impl.ts
index 74de308b1c0..50fe2efe1f6 100644
--- a/src/vs/workbench/api/node/extHost.api.impl.ts
+++ b/src/vs/workbench/api/node/extHost.api.impl.ts
@@ -738,6 +738,18 @@ export function createApiFactory(
 	};
 }
 
+/**
+ * Returns the original fs path (using the original casing for the drive letter)
+ */
+export function originalFSPath(uri: URI): string {
+	const result = uri.fsPath;
+	if (/^[a-zA-Z]:/.test(result) && uri.path.charAt(1).toLowerCase() === result.charAt(0)) {
+		// Restore original drive letter casing
+		return uri.path.charAt(1) + result.substr(1);
+	}
+	return result;
+}
+
 class Extension<T> implements vscode.Extension<T> {
 
 	private _extensionService: ExtHostExtensionService;
@@ -749,7 +761,7 @@ class Extension<T> implements vscode.Extension<T> {
 	constructor(extensionService: ExtHostExtensionService, description: IExtensionDescription) {
 		this._extensionService = extensionService;
 		this.id = description.id;
-		this.extensionPath = paths.normalize(description.extensionLocation.fsPath, true);
+		this.extensionPath = paths.normalize(originalFSPath(description.extensionLocation), true);
 		this.packageJSON = description;
 	}
 
diff --git a/src/vs/workbench/test/electron-browser/api/extHost.api.impl.test.ts b/src/vs/workbench/test/electron-browser/api/extHost.api.impl.test.ts
new file mode 100644
index 00000000000..12cbffb4ccf
--- /dev/null
+++ b/src/vs/workbench/test/electron-browser/api/extHost.api.impl.test.ts
@@ -0,0 +1,20 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import * as assert from 'assert';
+import URI from 'vs/base/common/uri';
+import { originalFSPath } from 'vs/workbench/api/node/extHost.api.impl';
+
+suite('ExtHost API', function () {
+	test('issue #51387: originalFSPath', function () {
+		assert.equal(originalFSPath(URI.file('C:\\test')).charAt(0), 'C');
+		assert.equal(originalFSPath(URI.file('c:\\test')).charAt(0), 'c');
+
+		assert.equal(originalFSPath(URI.revive(JSON.parse(JSON.stringify(URI.file('C:\\test'))))).charAt(0), 'C');
+		assert.equal(originalFSPath(URI.revive(JSON.parse(JSON.stringify(URI.file('c:\\test'))))).charAt(0), 'c');
+	});
+});