diff --git a/Foundation/NSPathUtilities.swift b/Foundation/NSPathUtilities.swift
index ab6dafd..4ffbff5 100644
--- a/Foundation/NSPathUtilities.swift
+++ b/Foundation/NSPathUtilities.swift
@@ -305,9 +305,8 @@
         if fixedSelf.length <= 1 {
             return fixedSelf
         }
-        
-        if let extensionPos = fixedSelf._startOfPathExtension {
-            return String(fixedSelf.characters.prefix(upTo: extensionPos))
+        if let extensionPos = (fixedSelf._startOfPathExtension) {
+            return String(fixedSelf.characters.prefix(upTo: extensionPos.predecessor()))
         } else {
             return fixedSelf
         }
@@ -318,7 +317,7 @@
             print("Cannot append extension \(str) to path \(self)")
             return nil
         }
-        let result = _swiftObject + str._stringByFixingSlashes(compress: false, stripTrailing: true)
+        let result = _swiftObject._stringByFixingSlashes(compress: false, stripTrailing: true) + "." + str
         return result._stringByFixingSlashes()
     }
 
diff --git a/TestFoundation/TestNSString.swift b/TestFoundation/TestNSString.swift
index 775858d..8eaa284 100644
--- a/TestFoundation/TestNSString.swift
+++ b/TestFoundation/TestNSString.swift
@@ -81,6 +81,8 @@
             ("test_stringByExpandingTildeInPath", test_stringByExpandingTildeInPath),
             ("test_stringByStandardizingPath", test_stringByStandardizingPath),
             ("test_stringByRemovingPercentEncoding", test_stringByRemovingPercentEncoding),
+            ("test_stringByAppendingPathExtension", test_stringByAppendingPathExtension),
+            ("test_stringByDeletingPathExtension", test_stringByDeletingPathExtension),
             ("test_ExternalRepresentation", test_ExternalRepresentation),
             ("test_mutableStringConstructor", test_mutableStringConstructor),
             ("test_PrefixSuffix", test_PrefixSuffix),
@@ -836,6 +838,36 @@
         XCTAssertNil(s2, "returns nil for a string with an invalid percent encoding")
     }
     
+    func test_stringByAppendingPathExtension() {
+        let values : Dictionary = [
+            NSString(string: "/tmp/scratch.old") : "/tmp/scratch.old.tiff",
+            NSString(string: "/tmp/scratch.") : "/tmp/scratch..tiff",
+            NSString(string: "/tmp/") : "/tmp.tiff",
+            NSString(string: "/scratch") : "/scratch.tiff",
+            NSString(string: "/~scratch") : "/~scratch.tiff",
+            NSString(string: "scratch") : "scratch.tiff",
+        ]
+        for (fileName, expectedResult) in values {
+            let result = fileName.stringByAppendingPathExtension("tiff")
+            XCTAssertEqual(result, expectedResult, "expected \(expectedResult) for \(fileName) but got \(result)")
+        }
+    }
+    
+    func test_stringByDeletingPathExtension() {
+        let values : Dictionary = [
+            NSString(string: "/tmp/scratch.tiff") : "/tmp/scratch",
+            NSString(string: "/tmp/") : "/tmp",
+            NSString(string: "scratch.bundle") : "scratch",
+            NSString(string: "scratch..tiff") : "scratch.",
+            NSString(string: ".tiff") : ".tiff",
+            NSString(string: "/") : "/",
+        ]
+        for (fileName, expectedResult) in values {
+            let result = fileName.stringByDeletingPathExtension
+            XCTAssertEqual(result, expectedResult, "expected \(expectedResult) for \(fileName) but got \(result)")
+        }
+    }
+    
     func test_ExternalRepresentation() {
         // Ensure NSString can be used to create an external data representation