Sending inline attachment from the Command Line

[Maarten van der Vlugt <maartenvandervlugt@xxxxxxxxxxxxxxx>]

I can not seem to set the disposition type of an attachment when  
sending from the command line. I would like to use mutt in a script to  
send scanned pdf files to a certain mailbox automatically so I use  
this command line:

mutt -a bon0001.pdf -s bon0001.pdf mail@xxxxxx < /dev/null

This results in an e-mail with an attachment declared like this:
Content-Disposition: attachment;
        filename=bon0001.pdf
Content-Type: application/pdf;
        name=bon0001.pdf
Content-Transfer-Encoding: base64

But I want "Content-Disposition: inline" for my pdf files so I can  
instantly read the files in my e-mail reader.

I can not seem to find the setting to get mutt to do this for me and  
it is driving me crazy! ;-) Who can help me?

Kind regards,

Maarten