Even though this post is 8+ years old, I believe it still applies: From CMIS how do I get the thumbnail placeholder for Alfresco?
Basically, CMIS would only return the thumbnail image and not the placeholder (which is what you want).
However, you may goto the mimetype image directly via extension:
Share:
http://localhost/share/components/images/filetypes/(extension)-file-48.png
Example:
http://localhost/share/components/images/filetypes/jpg-file-48.png
Alfresco:
http://localhost/alfresco/images/filetypes/(extension).png
Example:
http://localhost/alfresco/images/filetypes/jpg.png