Why do I see random text instead of my attachments?
That random text is likely your attachment, but you’re seeing the base64-encoded version of it because you are viewing the .eml or .mbox file in a plain text editor like TextEdit or Notepad.
Base64-encoding is the standard behind-the-scenes way of including attachments in a message. Most modern mail programs interpret this encoded information correctly and display the attachments either inline or as icons within the message. Simple text file viewers will not display these attachments correctly.
Try viewing or opening the file with an email application. Thunderbird and Apple Mail, for example, can open and view .eml files without you having to import anything.