[Enigmail] Cannot "Encrypt each attachment separately and send the message using inline PGP"

[ek]

Patrick Brunschwig @ 2008/02/14 00:33:
> ek wrote:
>> I'm trying to send a message with attachments and said option is 
>> grayed-out.  Only the first and third options are available.  And, well, 
>> I would like to use the second option.  Also, I assume there is a slight 
>> typo in this second option and it should read: Encrypt/sign [...] ?
> 
> The option is disabled in case the attachment is not a regular local
> file. E.g. if you forward a mail as attachment, attach links to web
> pages, or drag & drop an attachment from another email, then the option
> will be disabled.
> 


THanks for the info.  Can you explain why the option is disabled in 
these cases?