I found this working solution on Apple forum.
I tested it and it works great. Please pay attention to copy modfied qlgenerator in Library/QuickLook, NOT in System/Library/QuickLook
- Copy the "Illustrator.qlgenerator" from "/System/Library/QuickLook" to your desktop or any other folder you have read/write access to.
- Open "Illustrator.qlgenerator" by right clicking it and select "Show package contents".
- Open "Info.plist" in the "Contents" folder with a text editor.
- Find the line "<string>com.adobe.illustrator.ai-image</string>"
- Insert "<string>com.adobe.encapsulated-postscript</string>" below it with the same indentation and save the "Info.plist" file.
- Copy "Illustrator.qlgenerator" to "/Library/QuickLook"
If you did everything right, you should now have .EPS previews back again. That's a workaround until apple fixes it in the System/Library, hopefully soon.