I am having the very same issue (i.e. response containing the undefined url) and curious if this got resolved.

Endpoint GET https://developer.api.autodesk.com/bim360/rfis/v2/containers/:containerId/rfis/:rfiId/attachments

response: { "id": "cb3b7702-f65e-4a6e-bdc3-0ff5de43cc80", "name": "2fe417a1-18c7-4a3b-8460-ac7eecdf6cab.txt", "urn": "urn:adsk.wipprod:dm.lineage:yzt3AvZeSm69ww_13kPMgA", "urnType": "oss", "createdBy": "7262JZTKLJS27V8Q", "createdAt": "2025-02-10T16:20:25.000Z", "deletedBy": null, "deletedAt": null, "permittedActions": { "removeAttachment": true }, "url": "https://developer.api.autodesk.com/oss/v2/buckets/yzt3AvZeSm69ww_13kPMgA/objects/undefined" }

Thanks