Add note about environment variable in parameter file.html

war/src/main/webapp/help/parameter/file.html

@@ -22,6 +22,15 @@
     <code>my.zip</code>
     .) The name will not include the directory name portion.
   </p>
+  <p>
+    Please note that some shells (such as the default /bin/sh) may not be able
+    to recognize environment variables with special symbols like abc.zip. Try
+    using shebang like
+    <code>#!/bin/bash</code>
+    in shell script and use
+    <code>$(printenv abc.zip)</code>
+    to get the original file name.
+  </p>
   <p>
     File upload is optional. If a user chooses not to upload anything, Jenkins
     will simply skips this parameter and will not place anything (but it also