Finalize attachment

Finalize a previously uploaded seller case attachment.

This is step 2 of the case attachment flow.

Validation:

• imageId must match the expected URN shape for the case and attachment ID.
• Uploaded image must exist and no longer be in draft state.
• Case must belong to one of the authenticated seller’s orders.

Responses:

• Returns 400 for invalid IDs or unfinished uploads.
• Returns 404 when the case is missing/not owned.
• Returns the saved attachment descriptor on success.