How to set the date when linking an attachment to a work item using Azure Devops REST API? - azure-devops

Question

My team is moving to Azure Devops and I'm writing a custom tool to migrate all of the historical and current issues from our existing system, using the REST APIs of both products.
For this to be useful, it needs to be able to copy across all of the crucial data, including comments and attachments, with their correct dates and including inline links to images and other files.
Cloning a single work item at a time, I have currently got to the point where I have successfully:
1) Created the initial work item with basic info (description, created date, etc.)
2) Added all of the attachments that exist on the work item to Azure Devops
3) Linked the attachments to the work item
I now need to:
Add comments, and where necessary link to the already added attachments in the comment body.
The problem is with the dates of revisions that seem to get set by default when performing step 3 above.
If I try to add the comments after adding the attachments, because I am setting their createdDate in the past, I get an error telling me Dates must be increasing with each revision, which makes sense (somewhat) as adding each comment effectively creates a new revision of the work item.
I can add the comments before the attachments (which works fine as long as they are added chronologically), but at that point I obviously don't know the attachment ID as it hasn't been added yet.
I have tried passing an additional patch in the body of the request to link the attachment (as this works for the comments), but it seems to be ignored (just stabbing in the dark at this point as the docs are generally terrible).
I'll put the payload here as it illustrates the type of thing I want to do:
[
// this adds the actual file ✔️
{
"op":"add",
"path":"/relations/-",
"value": {
"rel":"AttachedFile",
"url":"https://dev.azure.com/FPipe/3e86b8a8-8108-4a35-86f3-4fb9ea599561/_apis/wit/attachments/1e1536de-dced-4455-a9ce-48e5a0e85ece?fileName=Drew%20Spencer.png",
"attributes": {
"comment":"Added from YouTrack"
}
}
},
// want to do something like this
{
"op":"add",
"path":"/fields/System.ChangedDate",
"value":"2021-05-16T16:54:37.076+01:00"
}
]
Is this possible?
If not does anybody know any decent workarounds?
I can think of two possible solutions:
Add the comments, then add and link the attachments, then update the comments with the correct links. Not great as it would require a second pass, and I'm not even sure it would work as the same issue could occur.
Just include the date from the old system in the comment body. If it comes to this, I'll do it, but I'd rather use the native features where possible.
There is a fragment on the sample in the docs with this patch, but there is literally 0 explanation about what it does:
{
"op": "test",
"path": "/rev",
"value": 3
},
I suspect something to do with revisions! The docs are genuinely terrible:
Source: https://learn.microsoft.com/en-us/rest/api/azure/devops/wit/work-items/update?view=azure-devops-rest-6.0&tabs=HTTP#operation
Thanks for your assistance.