Pierre from the ivanti support find the solution to my issue :
Defect 6329: "Attachment Collection created on new Module will not show in Mail Mapping for Incoming emails"
Fixed in 2016.2 but workaround for previous versions is:
1. Create a New module in Business Object, which module name is <Example Management>, top-level object name is <Example>.
2. Create an object Named <Example Attachments>(the object must include 'Attachment/Attachments' words), do not use the Object Creation Wizard else it doesn't work
3. Drag <Example> to <Example Attachments>, select "Yes" on the window.
4. In <Example>, a collection named <Example Attachments Collection> is created. Select <Example Attachments Collection> and click Manage Actions in Actions panel.
5. Create a Creation Action named <Add Attachment>, save the changes.
6. Create an available process of <Example> in Process Management, and activated it.
7. In Administration, Mail component, create a new mapping, select the new created process of Example in Process dropdown list.
8. In Business Attributes, the Attachments collection listed in the tree.