Fix a warning generated in return_bytes function and refactor it

It was multiplying a string containing a letter.

Moved function to Utils.php and display a human readable limit size

tpl/vintage/import.html

@@ -5,7 +5,7 @@
 <div id="pageheader">
   {include="page.header"}
   <div id="uploaddiv">
-    Import Netscape HTML bookmarks (as exported from Firefox/Chrome/Opera/Delicious/Diigo...) (Max: {$maxfilesize} bytes).
+    Import Netscape HTML bookmarks (as exported from Firefox/Chrome/Opera/Delicious/Diigo...) (Max: {$maxfilesize}).
     <form method="POST" action="?do=import" enctype="multipart/form-data"
           name="uploadform" id="uploadform">
       <input type="hidden" name="token" value="{$token}">