Make extension checks case-insensitive (#273)

## Summary

* Implement new `StringUtils::checkFileExtension` which does case
insensitive checking
* Move all checks over to this

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