diff --git a/src/activities/boot_sleep/SleepActivity.cpp b/src/activities/boot_sleep/SleepActivity.cpp
index 8d8fd79..72d74d7 100644
--- a/src/activities/boot_sleep/SleepActivity.cpp
+++ b/src/activities/boot_sleep/SleepActivity.cpp
@@ -9,20 +9,7 @@
 #include "CrossPointState.h"
 #include "fontIds.h"
 #include "images/CrossLarge.h"
-
-namespace {
-// Check if path has XTC extension (.xtc or .xtch)
-bool isXtcFile(const std::string& path) {
-  if (path.length() < 4) return false;
-  std::string ext4 = path.substr(path.length() - 4);
-  if (ext4 == ".xtc") return true;
-  if (path.length() >= 5) {
-    std::string ext5 = path.substr(path.length() - 5);
-    if (ext5 == ".xtch") return true;
-  }
-  return false;
-}
-}  // namespace
+#include "util/StringUtils.h"
 
 void SleepActivity::onEnter() {
   Activity::onEnter();
@@ -213,8 +200,8 @@ void SleepActivity::renderCoverSleepScreen() const {
 
   std::string coverBmpPath;
 
-  // Check if the current book is XTC or EPUB
-  if (isXtcFile(APP_STATE.openEpubPath)) {
+  if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtc") ||
+      StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".xtch")) {
     // Handle XTC file
     Xtc lastXtc(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastXtc.load()) {
@@ -228,7 +215,7 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     coverBmpPath = lastXtc.getCoverBmpPath();
-  } else {
+  } else if (StringUtils::checkFileExtension(APP_STATE.openEpubPath, ".epub")) {
     // Handle EPUB file
     Epub lastEpub(APP_STATE.openEpubPath, "/.crosspoint");
     if (!lastEpub.load()) {
@@ -242,6 +229,8 @@ void SleepActivity::renderCoverSleepScreen() const {
     }
 
     coverBmpPath = lastEpub.getCoverBmpPath();
+  } else {
+    return renderDefaultSleepScreen();
   }
 
   FsFile file;
diff --git a/src/activities/home/HomeActivity.cpp b/src/activities/home/HomeActivity.cpp
index b91e7c5..a774780 100644
--- a/src/activities/home/HomeActivity.cpp
+++ b/src/activities/home/HomeActivity.cpp
@@ -12,6 +12,7 @@
 #include "MappedInputManager.h"
 #include "ScreenComponents.h"
 #include "fontIds.h"
+#include "util/StringUtils.h"
 
 void HomeActivity::taskTrampoline(void* param) {
   auto* self = static_cast<HomeActivity*>(param);
@@ -44,10 +45,8 @@ void HomeActivity::onEnter() {
       lastBookTitle = lastBookTitle.substr(lastSlash + 1);
     }
 
-    const std::string ext4 = lastBookTitle.length() >= 4 ? lastBookTitle.substr(lastBookTitle.length() - 4) : "";
-    const std::string ext5 = lastBookTitle.length() >= 5 ? lastBookTitle.substr(lastBookTitle.length() - 5) : "";
     // If epub, try to load the metadata for title/author
-    if (ext5 == ".epub") {
+    if (StringUtils::checkFileExtension(lastBookTitle, ".epub")) {
       Epub epub(APP_STATE.openEpubPath, "/.crosspoint");
       epub.load(false);
       if (!epub.getTitle().empty()) {
@@ -56,9 +55,9 @@ void HomeActivity::onEnter() {
       if (!epub.getAuthor().empty()) {
         lastBookAuthor = std::string(epub.getAuthor());
       }
-    } else if (ext5 == ".xtch") {
+    } else if (StringUtils::checkFileExtension(lastBookTitle, ".xtch")) {
       lastBookTitle.resize(lastBookTitle.length() - 5);
-    } else if (ext4 == ".xtc") {
+    } else if (StringUtils::checkFileExtension(lastBookTitle, ".xtc")) {
       lastBookTitle.resize(lastBookTitle.length() - 4);
     }
   }
diff --git a/src/activities/network/CalibreWirelessActivity.cpp b/src/activities/network/CalibreWirelessActivity.cpp
index 3ac76cb..0ad9094 100644
--- a/src/activities/network/CalibreWirelessActivity.cpp
+++ b/src/activities/network/CalibreWirelessActivity.cpp
@@ -582,7 +582,7 @@ void CalibreWirelessActivity::handleSendBook(const std::string& data) {
 
   // Sanitize and create full path
   currentFilename = "/" + StringUtils::sanitizeFilename(filename);
-  if (currentFilename.find(".epub") == std::string::npos) {
+  if (!StringUtils::checkFileExtension(currentFilename, ".epub")) {
     currentFilename += ".epub";
   }
   currentFileSize = length;
diff --git a/src/activities/reader/FileSelectionActivity.cpp b/src/activities/reader/FileSelectionActivity.cpp
index af877a1..57a288c 100644
--- a/src/activities/reader/FileSelectionActivity.cpp
+++ b/src/activities/reader/FileSelectionActivity.cpp
@@ -5,6 +5,7 @@
 
 #include "MappedInputManager.h"
 #include "fontIds.h"
+#include "util/StringUtils.h"
 
 namespace {
 constexpr int PAGE_ITEMS = 23;
@@ -50,9 +51,8 @@ void FileSelectionActivity::loadFiles() {
       files.emplace_back(std::string(name) + "/");
     } else {
       auto filename = std::string(name);
-      std::string ext4 = filename.length() >= 4 ? filename.substr(filename.length() - 4) : "";
-      std::string ext5 = filename.length() >= 5 ? filename.substr(filename.length() - 5) : "";
-      if (ext5 == ".epub" || ext5 == ".xtch" || ext4 == ".xtc") {
+      if (StringUtils::checkFileExtension(filename, ".epub") || StringUtils::checkFileExtension(filename, ".xtch") ||
+          StringUtils::checkFileExtension(filename, ".xtc")) {
         files.emplace_back(filename);
       }
     }
diff --git a/src/activities/reader/ReaderActivity.cpp b/src/activities/reader/ReaderActivity.cpp
index 1829218..cb123e1 100644
--- a/src/activities/reader/ReaderActivity.cpp
+++ b/src/activities/reader/ReaderActivity.cpp
@@ -6,6 +6,7 @@
 #include "Xtc.h"
 #include "XtcReaderActivity.h"
 #include "activities/util/FullScreenMessageActivity.h"
+#include "util/StringUtils.h"
 
 std::string ReaderActivity::extractFolderPath(const std::string& filePath) {
   const auto lastSlash = filePath.find_last_of('/');
@@ -16,14 +17,7 @@ std::string ReaderActivity::extractFolderPath(const std::string& filePath) {
 }
 
 bool ReaderActivity::isXtcFile(const std::string& path) {
-  if (path.length() < 4) return false;
-  std::string ext4 = path.substr(path.length() - 4);
-  if (ext4 == ".xtc") return true;
-  if (path.length() >= 5) {
-    std::string ext5 = path.substr(path.length() - 5);
-    if (ext5 == ".xtch") return true;
-  }
-  return false;
+  return StringUtils::checkFileExtension(path, ".xtc") || StringUtils::checkFileExtension(path, ".xtch");
 }
 
 std::unique_ptr<Epub> ReaderActivity::loadEpub(const std::string& path) {
diff --git a/src/util/StringUtils.cpp b/src/util/StringUtils.cpp
index 2161721..e296d37 100644
--- a/src/util/StringUtils.cpp
+++ b/src/util/StringUtils.cpp
@@ -1,5 +1,7 @@
 #include "StringUtils.h"
 
+#include <cstring>
+
 namespace StringUtils {
 
 std::string sanitizeFilename(const std::string& name, size_t maxLength) {
@@ -33,4 +35,18 @@ std::string sanitizeFilename(const std::string& name, size_t maxLength) {
   return result.empty() ? "book" : result;
 }
 
+bool checkFileExtension(const std::string& fileName, const char* extension) {
+  if (fileName.length() < strlen(extension)) {
+    return false;
+  }
+
+  const std::string fileExt = fileName.substr(fileName.length() - strlen(extension));
+  for (size_t i = 0; i < fileExt.length(); i++) {
+    if (tolower(fileExt[i]) != tolower(extension[i])) {
+      return false;
+    }
+  }
+  return true;
+}
+
 }  // namespace StringUtils
diff --git a/src/util/StringUtils.h b/src/util/StringUtils.h
index 27f826a..f846cf1 100644
--- a/src/util/StringUtils.h
+++ b/src/util/StringUtils.h
@@ -1,4 +1,5 @@
 #pragma once
+
 #include <string>
 
 namespace StringUtils {
@@ -10,4 +11,9 @@ namespace StringUtils {
  */
 std::string sanitizeFilename(const std::string& name, size_t maxLength = 100);
 
+/**
+ * Check if the given filename ends with the specified extension (case-insensitive).
+ */
+bool checkFileExtension(const std::string& fileName, const char* extension);
+
 }  // namespace StringUtils