We should open an issue for this [you already did] but it’s not clear exactly how we would fix it. Enketo and Collect will send multiple POST requests, where each has the same submission XML and a subset of the attachments for that submission, when the attachment sizes get large enough. This is part of the OpenRosa specification that KoboToolbox and other software in the ODK ecosystem follow. Some details about this multiple-POST process are here:
Should we delay contacting the external REST Services endpoint until all media files are received for a particular submission? That doesn’t really seem right: there’s a chance that all those files will never be received.
I think your best option for a quick workaround is going to be polling the KoboToolbox data API until all the attachments appear. You could initiate the polling once REST Services has made you aware of a submission. It would be up to you to decide how long you want to keep retrying before giving up on receipt of any missing attachments.
cc @nolive
