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.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