.eps and .ai-files does not work in Apple Mail

It's odd since you have Illustrator installed you can't see the content. Try selecting one of the .ai files, press "command + i" and check what is set to open them…

Mine is set to A.D. because I don't have Illustrator but since you do it should be set to Illustrator. If it's not change it in the drop down menu and click "Change All…"

FWIW, I only have a few .eps files but Preview opens all the ones I have. The content also shows in the Finder preview pane. I don't know that means any .eps file will work. It may depend on what created them.

.ai files are Adobe Illustrator files. I have 57 of those, some display, some don't. Some seem to be support files such as patterns and some are actual graphics files.

Do you have Adobe Illustrator?

FWIW, neither .eps not .ai attachments display in Mail. However you can see them using "Quick Look Attachments" from the File menu or by pressing "Command + Y". Whether they display for the recipient would depend on their email client.

I'm experiencing something similar - just started with my upgrade to Sierra. Colleagues send AI file attachment and I get a PDF icon labeled "Mail Attachment.as" - it cannot be opened.. even changing the extension to AI.. I can move the file to the desktop and it shows it's zero bytes. Attempting to double-click on it does nothing. Well not entirely, Apple thinks it's a zip file and it creates a second file called "Mail Attachment.as.cpgz" then when you double-click that file it creates "Mail Attachment 2.as" - this cycle can continue indefinitely. (but NO AI file..) Oddly enough I can send an AI file to myself and it works fine. (I had the colleague test her computer and apparently it's fine on her end. - she's using El Capitan)

We've experienced the same issue using Exchange mail server in our office.

It occasionally occurs when we receive Illustrator files from customers. The customers sends the Illustrator file to the sales person (which comes over correctly as an .ai file) and when the sales person forwards the file to me from their Windows computer, I receive a .as file, which I cannot open. The solution for us has been for the sales person to download the Illustrator file to their computer first, reattach the file and then send it directly to me. That seems to fix the problem, although its a bit of an inconvenience. We have not figured out the exact cause, but the suspicion has been that its the Exchange mail server and/or the Mac/Windows compatibility using Apple Mail when attachments are forwarded. I'm not knowledgeable enough about operating systems to troubleshoot what causes this problem to happen.

@RedCrayonDesign - I was having the same .as file issue when I was running OS Mavericks and I am now running Sierra and still have the same issue.

Are these files you are receiving as attachments?

Can you control-click, and choose "Save"?

Do you have any problems with other types of files?

You can see them - you mean actually see their contents inside the message, or just see that they're there?

FWIW, I meant select one of the .ai files in the Finder then press "command + i". You should get an info panel.

I am just experiencing this issue for the first time.Anyone have an amswer? Exact same issue as you are describing RedCrayon!