sylpheed --compose + attachment??

[Johan Spee]

In sylpheed a new message to john at home can be opened from the command line like this:
# sylpheed --compose john at home

I would like to do something similar with an attachment. But when I try this:
# sylpheed --compose somefile.txt
the 'somefile.txt' part is interpreted as an address.

I cannot find any information on other command-line parameters for sylpheed. Can this be done?

thanks,

-- 
Johan Spee