Index: src/OFString+PathAdditions.h
==================================================================
--- src/OFString+PathAdditions.h
+++ src/OFString+PathAdditions.h
@@ -75,14 +75,22 @@
  * @param component The path component to append
  * @return A new, autoreleased OFString with the path component appended
  */
 - (OFString *)stringByAppendingPathComponent: (OFString *)component;
 
+/**
+ * @brief Creates a new string by appending a path extension.
+ *
+ * @param extension The extension to append
+ * @return A new, autoreleased OFString with the path extension appended
+ */
+- (OFString *)stringByAppendingPathExtension: (OFString *)extension;
+
 - (bool)of_isDirectoryPath;
 - (OFString *)of_pathToURLPathWithURLEncodedHost:
     (OFString *__autoreleasing _Nullable *_Nonnull)URLEncodedHost;
 - (OFString *)of_URLPathToPathWithURLEncodedHost:
     (nullable OFString *)URLEncodedHost;
 - (OFString *)of_pathComponentToURLPathComponent;
 @end
 
 OF_ASSUME_NONNULL_END

Index: src/platform/AmigaOS/OFString+PathAdditions.m
==================================================================
--- src/platform/AmigaOS/OFString+PathAdditions.m
+++ src/platform/AmigaOS/OFString+PathAdditions.m
@@ -266,10 +266,31 @@
 		[ret makeImmutable];
 
 		return ret;
 	}
 }
+
+- (OFString *)stringByAppendingPathExtension: (OFString *)extension
+{
+	if ([self hasSuffix: @"/"]) {
+		void *pool = objc_autoreleasePoolPush();
+		OFMutableArray *components;
+		OFString *fileName, *ret;
+
+		components =
+		    [[self.pathComponents mutableCopy] autorelease];
+		fileName = [components.lastObject
+		    stringByAppendingFormat: @".%@", extension];
+		[components replaceObjectAtIndex: components.count - 1
+				      withObject: fileName];
+
+		ret = [[OFString pathWithComponents: components] retain];
+		objc_autoreleasePoolPop(pool);
+		return [ret autorelease];
+	} else
+		return [self stringByAppendingFormat: @".%@", extension];
+}
 
 - (bool)of_isDirectoryPath
 {
 	return ([self hasSuffix: @"/"] || [self hasSuffix: @":"] ||
 	    [OFFileURLHandler of_directoryExistsAtPath: self]);

Index: src/platform/POSIX/OFString+PathAdditions.m
==================================================================
--- src/platform/POSIX/OFString+PathAdditions.m
+++ src/platform/POSIX/OFString+PathAdditions.m
@@ -311,10 +311,31 @@
 		[ret makeImmutable];
 
 		return ret;
 	}
 }
+
+- (OFString *)stringByAppendingPathExtension: (OFString *)extension
+{
+	if ([self hasSuffix: @"/"]) {
+		void *pool = objc_autoreleasePoolPush();
+		OFMutableArray *components;
+		OFString *fileName, *ret;
+
+		components =
+		    [[self.pathComponents mutableCopy] autorelease];
+		fileName = [components.lastObject
+		    stringByAppendingFormat: @".%@", extension];
+		[components replaceObjectAtIndex: components.count - 1
+				      withObject: fileName];
+
+		ret = [[OFString pathWithComponents: components] retain];
+		objc_autoreleasePoolPop(pool);
+		return [ret autorelease];
+	} else
+		return [self stringByAppendingFormat: @".%@", extension];
+}
 
 - (bool)of_isDirectoryPath
 {
 	return ([self hasSuffix: @"/"] ||
 	    [OFFileURLHandler of_directoryExistsAtPath: self]);

Index: src/platform/Windows/OFString+PathAdditions.m
==================================================================
--- src/platform/Windows/OFString+PathAdditions.m
+++ src/platform/Windows/OFString+PathAdditions.m
@@ -316,10 +316,31 @@
 		[ret makeImmutable];
 
 		return ret;
 	}
 }
+
+- (OFString *)stringByAppendingPathExtension: (OFString *)extension
+{
+	if ([self hasSuffix: @"\\"] || [self hasSuffix: @"/"]) {
+		void *pool = objc_autoreleasePoolPush();
+		OFMutableArray *components;
+		OFString *fileName, *ret;
+
+		components =
+		    [[self.pathComponents mutableCopy] autorelease];
+		fileName = [components.lastObject
+		    stringByAppendingFormat: @".%@", extension];
+		[components replaceObjectAtIndex: components.count - 1
+				      withObject: fileName];
+
+		ret = [[OFString pathWithComponents: components] retain];
+		objc_autoreleasePoolPop(pool);
+		return [ret autorelease];
+	} else
+		return [self stringByAppendingFormat: @".%@", extension];
+}
 
 - (bool)of_isDirectoryPath
 {
 	return ([self hasSuffix: @"\\"] || [self hasSuffix: @"/"] ||
 	    [OFFileURLHandler of_directoryExistsAtPath: self]);

Index: src/platform/libfat/OFString+PathAdditions.m
==================================================================
--- src/platform/libfat/OFString+PathAdditions.m
+++ src/platform/libfat/OFString+PathAdditions.m
@@ -312,10 +312,31 @@
 		[ret makeImmutable];
 
 		return ret;
 	}
 }
+
+- (OFString *)stringByAppendingPathExtension: (OFString *)extension
+{
+	if ([self hasSuffix: @"/"]) {
+		void *pool = objc_autoreleasePoolPush();
+		OFMutableArray *components;
+		OFString *fileName, *ret;
+
+		components =
+		    [[self.pathComponents mutableCopy] autorelease];
+		fileName = [components.lastObject
+		    stringByAppendingFormat: @".%@", extension];
+		[components replaceObjectAtIndex: components.count - 1
+				      withObject: fileName];
+
+		ret = [[OFString pathWithComponents: components] retain];
+		objc_autoreleasePoolPop(pool);
+		return [ret autorelease];
+	} else
+		return [self stringByAppendingFormat: @".%@", extension];
+}
 
 - (bool)of_isDirectoryPath
 {
 	return ([self hasSuffix: @"/"] ||
 	    [OFFileURLHandler of_directoryExistsAtPath: self]);

Index: tests/OFStringTests.m
==================================================================
--- tests/OFStringTests.m
+++ tests/OFStringTests.m
@@ -593,10 +593,28 @@
 	TEST(@"-[stringByAppendingPathComponent:]",
 	    [[mutableString1 stringByAppendingPathComponent: @"baz"]
 	    isEqual: mutableString2] &&
 	    [[string stringByAppendingPathComponent: @"baz"]
 	    isEqual: mutableString2])
+
+# if defined(OF_WINDOWS) || defined(OF_MSDOS)
+	TEST(@"-[stringByAppendingPathExtension:]",
+	    [[C(@"foo") stringByAppendingPathExtension: @"bar"]
+	    isEqual: @"foo.bar"] &&
+	    [[C(@"c:\\tmp\\foo") stringByAppendingPathExtension: @"bar"]
+	    isEqual: @"c:\\tmp\\foo.bar"] &&
+	    [[C(@"c:\\tmp\\/\\") stringByAppendingPathExtension: @"bar"]
+	    isEqual: @"c:\\tmp.bar"])
+# else
+	TEST(@"-[stringByAppendingPathExtension:]",
+	    [[C(@"foo") stringByAppendingPathExtension: @"bar"]
+	    isEqual: @"foo.bar"] &&
+	    [[C(@"foo/bar") stringByAppendingPathExtension: @"baz"]
+	    isEqual: @"foo/bar.baz"] &&
+	    [[C(@"foo///") stringByAppendingPathExtension: @"bar"]
+	    isEqual: @"foo.bar"])
+# endif
 #endif
 
 	TEST(@"-[hasPrefix:]", [C(@"foobar") hasPrefix: @"foo"] &&
 	    ![C(@"foobar") hasPrefix: @"foobar0"])