rails 3 Mail encoding issue

[Rails 3 - Ruby 1.9.2]
I have to get email attachments ( mp4 files ) and store them...
I get easily all parts using Mail ( .... <Content-Type: multipart/
mixed...."> )
        attachment = mail.attachments[0]
        #<Mail::Part:2206885940, Multipart: false, Headers: <Content-
Type: video/mp4; name=landscape.mp4>, <Content-Transfer-Encoding:
base64>, <Content-Disposition: attachment; filename=landscape.mp4>>
got what I need,
but trying to store this attachment in a temp file , I get an
encoding
error :
     File.open(filepath,File::CREAT|File::TRUNC|File::WRONLY,0644)
{ |
f> f.write(attachment.body) }
       Encoding::UndefinedConversionError Exception: "\x80" from
ASCII-8BIT to UTF-8
did I miss anything before writing the file ? or is it a known Ruby
bug ?
being an .mp4 attachment, it should be a binary file.. it comes as an
UTF-8 ( Base64) ? , right...
how should I write it as a binary too ?
tfyh