diff --git a/src/doc/user/usermanual.xml b/src/doc/user/usermanual.xml
index 8fa9472c..70861465 100644
--- a/src/doc/user/usermanual.xml
+++ b/src/doc/user/usermanual.xml
@@ -6021,13 +6021,18 @@ x-my-tag = mailmytag
 
         <para><filename>mimemap</filename> specifies the
         file name extension to MIME type mappings.</para> 
-
+      
         <para>For file names without an extension, or with an unknown one,
         a system command (<command>file</command> <option>-i</option>, or
         <command>xdg-mime</command>) will be executed to determine the MIME
         type (this can be switched off, or the command changed inside the
         main configuration file).</para>
 
+        <para>All extension values in <filename>mimemap</filename> must be
+        entered in lower case. File names extensions are lower-cased for
+        comparison during indexing, meaning that an upper case
+        <filename>mimemap</filename> entry will never be matched.</para>
+
         <para>The mappings can be specified on a per-subtree basis,
         which may be useful in some cases. Example:
         <application>okular</application> notes have a
diff --git a/src/sampleconf/mimemap b/src/sampleconf/mimemap
index 50fb4d21..783a6b05 100644
--- a/src/sampleconf/mimemap
+++ b/src/sampleconf/mimemap
@@ -1,5 +1,9 @@
 # (C) 2004 J.F.Dockes
 # Associations of file name extensions to mime types
+#
+# All entries must be in lower case characters. File name extensions are
+# lower-cased for comparison during indexing, meaning that an upper or
+# mixed case entry will never be matched.
 
 .txt = text/plain
 .text = text/plain