Large attachment in S/MIME

We are actually looking at using S/MIME in a file manager (Windows Explorer,
...) but we are not aware of any RFC or drafts describing this process.
Has anybody seen anything about this?

Another problem we are trying to solve concerns very large file (or attachment
when it is a message) in S/MIME. As the ContentInfo need to be DER encoded, it
is not possible to process a buffer excessively large. What should be done in
this case?

Thanks for your help.