diff --git a/solr/core/src/java/org/apache/solr/util/SimplePostTool.java b/solr/core/src/java/org/apache/solr/util/SimplePostTool.java
index f2d9ef5dc55..efbd2fb85fa 100644
--- a/solr/core/src/java/org/apache/solr/util/SimplePostTool.java
+++ b/solr/core/src/java/org/apache/solr/util/SimplePostTool.java
@@ -25,6 +25,7 @@ import java.io.InputStream;
 import java.io.ByteArrayInputStream;
 import java.io.OutputStream;
 import java.io.UnsupportedEncodingException;
+import java.util.Locale;
 import java.util.HashMap;
 import java.util.Set;
 import java.util.HashSet;
@@ -392,7 +393,7 @@ public class SimplePostTool {
   private String guessType(File file) {
     String name = file.getName();
     String suffix = name.substring(name.lastIndexOf(".")+1);
-    return mimeMap.get(suffix.toLowerCase());
+    return mimeMap.get(suffix.toLowerCase(Locale.ROOT));
   }
 
   /**
@@ -570,4 +571,4 @@ public class SimplePostTool {
       return p.matcher(file.getName()).find();
     }
   }
-}
\ No newline at end of file
+}