Custom rule don't match without empty line before the string!

[saqariden]

Hello guys,

I have the following SA rule which is supposed to block base64 encoded 
mails:


body                EN_BASE64_B        /(Content-Transfer-Encoding: 
base64\sContent-Type: text\/(plain|html); 
charset="?utf-8"?)|(Content-Type: text\/(plain|html); 
charset="?utf-8"?\sContent-Transfer-Encoding: base64)/i
describe             EN_BASE64_B        TEXT OR HTML B64 ENCODED
score                 EN_BASE64_B        5

this is the mail that i want to stop:

--------------------------
(... header header...)
X-Scanned-By: MIMEDefang 2.79 # last header line here


--Boundary_(ID_xxxxxx)
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64

SGVsbG8hIA0KDQpIYXZlIHlvdSAgc2VlbiAgdGhpcz8gIFlvdSd2ZSBnb3Qg
dG8gdGFrZSAgYSBsb29rLCB0aGF0J3MgYXdlc29tZSwgcmVhZGluZyAgbW9y
ZSBoZXJlIGh0dHA6Ly93d3cuZHVydXNhbGlzLmx0L2JjaC5waHA/a3lnDQoN=

--Boundary_(ID_IcM7JgudOk13I7fXE7a3Zw)--
-------------------------

the rule don't match for this mail, but it match when i had an empty 
line like this:

--Boundary_(ID_xxxxxx)
   #empty line here
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64

How can i do to match the both, with the empty line and without it? THANK'S

Regards,
S.AQARIDEN.
Signature Academique