You are using Case Feed with Email to Case Premium / Case Merge Premium and notice that inline images included on inbound emails result in an error of "This file is no longer available".

Salesforce delivered the ability to render inline images in Case Feed for standard Email-to-Case users with the Summer '15 release.

Unfortunately, there is a current Salesforce platform limitation that prevents inline images from being rendered in Case Feed for emails processed through a custom apex email service, such as Email to Case Premium, and emails recreated using apex, such as the child record cloning process for Case Merge Premium.

We're encouraging all Email to Case Premium & Case Merge Premium customers to please review and up-vote the following Success Community idea, asking Salesforce to add a mechanism to handle this.
https://success.salesforce.com/ideaView?id=08730000000wkQpAAI