You'll need 2 sections. Each section has its own attachment field.
Section1 default section
Section 2 - "Read Only"
You'll also need a read-only field i.e check box/hidden text box etc with a default value of 0 i.e. hide
On the attachment field which is inside Section 2, select the attachment field properties
Data>uncheck Allow users to browse,delete etc.
Then each section has a formatting rule to hide it depending on the value of the 'hide' filed
Section1 condition hide = 1 - select Hide this Control
Section 2 condition hide = 0 - select Hide this Control
Attached is an example.