fs: accept all valid utf8 values in fast paths

lib/fs.js

@@ -429,7 +429,8 @@ function tryReadSync(fd, isUserFd, buffer, pos, len) {
 function readFileSync(path, options) {
   options = getOptions(options, { flag: 'r' });
 
-  if (options.encoding === 'utf8' || options.encoding === 'utf-8') {
+  if (options.encoding === 'utf8' || options.encoding === 'utf-8' ||
+      options.encoding === 'UTF8' || options.encoding === 'UTF-8') {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }
@@ -2390,7 +2391,8 @@ function writeFileSync(path, data, options) {
   const flag = options.flag || 'w';
 
   // C++ fast path for string data and UTF8 encoding
-  if (typeof data === 'string' && (options.encoding === 'utf8' || options.encoding === 'utf-8')) {
+  if (typeof data === 'string' && (options.encoding === 'utf8' || options.encoding === 'utf-8' ||
+      options.encoding === 'UTF8' || options.encoding === 'UTF-8')) {
     if (!isInt32(path)) {
       path = getValidatedPath(path);
     }

test/parallel/test-fs-utf8-fast-path-casing.js

@@ -0,0 +1,49 @@
+'use strict';
+
+// This test ensures that fs.readFileSync and fs.writeFileSync
+// accept all valid UTF8 encoding variants (utf8, utf-8, UTF8, UTF-8).
+// Refs: https://github.com/nodejs/node/issues/49888
+
+const common = require('../common');
+const tmpdir = require('../../test/common/tmpdir');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+
+tmpdir.refresh();
+
+const testContent = 'Hello, World! 你好，世界！';
+const encodings = ['utf8', 'utf-8', 'UTF8', 'UTF-8'];
+
+// Test writeFileSync and readFileSync with different UTF8 variants
+for (const encoding of encodings) {
+  const testFile = tmpdir.resolve(`test_utf8_fast_path_${encoding}.txt`);
+
+  // Test writeFileSync
+  fs.writeFileSync(testFile, testContent, { encoding });
+
+  // Test readFileSync
+  const result = fs.readFileSync(testFile, { encoding });
+  assert.strictEqual(result, testContent,
+    `readFileSync should return correct content for encoding "${encoding}"`);
+}
+
+// Test with file descriptor
+for (const encoding of encodings) {
+  const testFile = tmpdir.resolve(`test_utf8_fast_path_fd_${encoding}.txt`);
+
+  // Write with fd
+  const fdWrite = fs.openSync(testFile, 'w');
+  fs.writeFileSync(fdWrite, testContent, { encoding });
+  fs.closeSync(fdWrite);
+
+  // Read with fd
+  const fdRead = fs.openSync(testFile, 'r');
+  const result = fs.readFileSync(fdRead, { encoding });
+  fs.closeSync(fdRead);
+
+  assert.strictEqual(result, testContent,
+    `readFileSync with fd should return correct content for encoding "${encoding}"`);
+}
+
+console.log('All UTF8 fast path casing tests passed!');