There is no way to do that.
Show original is Gmail's way to let you see (and download the EML) of the full email source with the headers from emails it received as an email.
An EML attachment isn't an email Gmail received, but a simple attachment and is viewed like any attachment (with a preview based on the format) hence it doesn't include any "email" tools from Gmail.
You should download the EML and view in a text editor (which will show you the source of the email like you'd see in, the bottom part of, Show original). If you want this top part parsing like the Show original page has, you can try to paste in the content of the EML in Google Admin Toolbox's Messageheader tool, which has a similar look.