Constructing MIME message without loading message stream

[Kris Kennaway]

I would like to MIME encode a message from a large file without first 
loading the file into memory.  Assume the file has been pre-encoded on 
disk (actually I am using encode_7or8bit, so the encoding should be 
null).  Is there a way to construct the flattened MIME message such that 
data is streamed from the file as needed instead of being resident in 
memory?  Do I have to subclass the MIMEBase class myself?

Kris