Remove save extension check for images because it can cause problems when

Remove save extension check for images because it can cause problems when
the extension is not save and the image url points to the url base address.
If the server uses session trans_sid then somehow the session is destroyed
because of the automatic ?SID=session_id_string.
At least that's what I think what happens.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@3893 7612ce4b-ef26-0410-bec9-ea0150e637f0