fixed file extension for offline mode player data

added ".dat" file extension so this is able to find a players offline mode data if available

CraftBukkit-Patches/0126-Use-Offline-Player-Data-Once-if-Required.patch

@@ -17,7 +17,7 @@ index 93ff8d3..cc4d487 100644
 +            boolean usingWrongFile = false;
 +            if ( !file1.exists() )
 +            {
-+                file1 = new File( this.playerDir, UUID.nameUUIDFromBytes( ( "OfflinePlayer:" + entityhuman.getName() ).getBytes( "UTF-8" ) ).toString() );
++                file1 = new File( this.playerDir, UUID.nameUUIDFromBytes( ( "OfflinePlayer:" + entityhuman.getName() ).getBytes( "UTF-8" ) ).toString() + ".dat");
 +                if ( file1.exists() )
 +                {
 +                    usingWrongFile = true;