Not able to create Trigger on Attachment from Salesforce UI
Salesforce doesn't have the option to allow developers to create triggers directly on standard objects like Attachment using the Salesforce UI. Instead, we've used tools Force.com IDE in Eclipse to write and deploy triggers on Attachment. For the Attachment, ContentDocument, and Note standard objects, you can’t create a trigger in the Salesforce user interface. For these objects, create a trigger using development tools, such as the Developer Console or the Salesforce extensions for Visual Studio Code. Alternatively, you can also use the Metadata API. Here's an example