Discussion of the MIME part’s encoding as being an inefficient size is missing the forest for the trees.

The entire message is (or can be) compressed before transmission (eg. When IMAP has DEFLATE enabled).

Just because an intermediate encoding step expands binary to make it text safe doesn’t mean it has to stay uncompressed during the entire existence of that MIMe message.

If all you need is file transfer even the message header is a lot of overhead (how much overhead depends on the client and how many devices handle the message). Mail servers don't always handle large files very well either. Even if they upload correctly downloading can be difficult. It's not uncommon for a single message with a large attachment to clog a mailbox and prevent other messages from being sent/received. That said, I'm not even saying it can't/wont work, just that there's better options for sending files and there are certainly better MUAs than outlook.