diff --git a/include/openpose/utilities/string.hpp b/include/openpose/utilities/string.hpp
index 596329cc..16df3134 100644
--- a/include/openpose/utilities/string.hpp
+++ b/include/openpose/utilities/string.hpp
@@ -16,6 +16,10 @@ namespace op
     OP_API std::string toFixedLengthString(const T number, const unsigned long long stringLength = 0);
 
     OP_API std::vector<std::string> splitString(const std::string& stringToSplit, const std::string& delimiter);
+
+    OP_API std::string toLower(const std::string& string);
+
+    OP_API std::string toUpper(const std::string& string);
 }
 
 #endif // OPENPOSE_UTILITIES_STRING_HPP
diff --git a/src/openpose/producer/imageDirectoryReader.cpp b/src/openpose/producer/imageDirectoryReader.cpp
index bf5cd5ff..12df26f4 100644
--- a/src/openpose/producer/imageDirectoryReader.cpp
+++ b/src/openpose/producer/imageDirectoryReader.cpp
@@ -10,8 +10,8 @@ namespace op
         try
         {
             // Get files on directory with the desired extensions
-            const std::vector<std::string> extensions{".bmp", ".dib", ".pbm", ".pgm", ".ppm", ".sr", ".ras",     // Completely supported by OpenCV
-                                                      ".jpg", "jpeg", ".png"};                                   // Most of them supported by OpenCV
+            const std::vector<std::string> extensions{"bmp", "dib", "pbm", "pgm", "ppm", "sr", "ras",   // Completely supported by OpenCV
+                                                      "jpg", "jpeg", "png"};                            // Most of them supported by OpenCV
             const auto imagePaths = getFilesOnDirectory(imageDirectoryPath, extensions);
 
             // Check #files > 0
diff --git a/src/openpose/utilities/fileSystem.cpp b/src/openpose/utilities/fileSystem.cpp
index 3cc87f5f..67a02ba3 100644
--- a/src/openpose/utilities/fileSystem.cpp
+++ b/src/openpose/utilities/fileSystem.cpp
@@ -1,6 +1,7 @@
 #include <boost/filesystem.hpp>
 #include <boost/range/iterator_range_core.hpp>
 #include <openpose/utilities/fileSystem.hpp>
+#include <openpose/utilities/string.hpp>
 
 namespace op
 {
@@ -134,9 +135,9 @@ namespace op
     {
         try
         {
-            const auto cleanedExtension = removeExtensionDot(extension);
+            const auto cleanedExtension = toLower(removeExtensionDot(extension));
             for (auto& extensionI : extensions)
-                if (cleanedExtension == removeExtensionDot(extensionI))
+                if (cleanedExtension == toLower(removeExtensionDot(extensionI)))
                     return true;
             return false;
         }
diff --git a/src/openpose/utilities/string.cpp b/src/openpose/utilities/string.cpp
index 31613c2e..a15857b0 100644
--- a/src/openpose/utilities/string.cpp
+++ b/src/openpose/utilities/string.cpp
@@ -1,3 +1,4 @@
+#include <algorithm> // std::transform
 #include <openpose/utilities/string.hpp>
 
 namespace op
@@ -55,6 +56,36 @@ namespace op
         }
     }
 
+    std::string toLower(const std::string& string)
+    {
+        try
+        {
+            auto result = string;
+            std::transform(string.begin(), string.end(), result.begin(), tolower);
+            return result;
+        }
+        catch (const std::exception& e)
+        {
+            error(e.what(), __LINE__, __FUNCTION__, __FILE__);
+            return "";
+        }
+    }
+
+    std::string toUpper(const std::string& string)
+    {
+        try
+        {
+            auto result = string;
+            std::transform(string.begin(), string.end(), result.begin(), toupper);
+            return result;
+        }
+        catch (const std::exception& e)
+        {
+            error(e.what(), __LINE__, __FUNCTION__, __FILE__);
+            return "";
+        }
+    }
+
 
     // Signed
     template std::string toFixedLengthString<char>(const char number, const unsigned long long stringLength);