implement EXISTING_FILE_VALUE type handler

when the user pass option type FileInputStream.class, I think the expected behavior for the return value is the same type, which the user passed.
Before this there was no check whether the file exist.

src/main/java/org/apache/commons/cli/TypeHandler.java

@@ -18,6 +18,7 @@
 package org.apache.commons.cli;
 
 import java.io.File;
+import java.io.FileInputStream;
 
 import java.net.MalformedURLException;
 import java.net.URL;
@@ -87,7 +88,7 @@ else if (PatternOptionBuilder.FILE_VALUE == clazz)
         }
         else if (PatternOptionBuilder.EXISTING_FILE_VALUE == clazz)
         {
-            return createFile(str);
+            return openFile(str);
         }
         else if (PatternOptionBuilder.FILES_VALUE == clazz)
         {
@@ -222,6 +223,24 @@ public static File createFile(String str)
         return new File(str);
     }
 
+    /**
+     * Returns the opened FileInputStream represented by <code>str</code>.
+     *
+     * @param str the file location
+     * @return The file input stream represented by <code>str</code>.
+     */
+    public static FileInputStream openFile(String str) throws ParseException
+    {
+        try
+        {
+            return new FileInputStream(str);
+        }
+        catch (FileNotFoundException e)
+        {
+            throw new ParseException("Unable to find file: " + str);
+        }
+    }
+
     /**
      * Returns the File[] represented by <code>str</code>.
      * <p>