It is not possible to batch add or update attachments to a feature service in ArcGIS Online, and there is no direct way to accomplish this workflow. Users must add or update the attachments in ArcGIS Online one at a time in editing mode.
However, it is possible to add multiple attachments to a feature class using the Generate Attachment Match Table tool and the Add Attachments tool in ArcMap. Follow the steps below:
The Generate Attachment Match Table and Add Attachments tools are only available with ArcGIS Desktop Standard or Advanced licenses.
Alternatively, a Python script can use the Add Attachment and Update Attachment operations on the REST API to add and update attachments.
- In ArcGIS Online, download the feature service to a file geodatabase without any of the attachments.
- Follow the process described in the following knowledgebase article, How To: Add multiple documents as attachments to a feature class, to re-add all the attachments.
- Once the previous step is done, publish the service to ArcGIS Online as a new service or overwrite the existing service.