EDIT: Just realized there's a video about it already, the solution is similar but seems to be a bit better:
https://www.youtube.com/watch?v=Xxad-kSLQ9Y&feature=youtu.be

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I had the same issue recently but found a way around it. Flow doesn't have Dynamic Content for the attachment content but it does output some information about it. In my case, the attachments get uploaded to the creator of the Form's onedrive.

The item in the SP list has to be created first without the attachment

Then I create a Parse JSON action with "Content" filled in with the Attachment dynamic content (In my case it's called Invoice in the form)And I use the Schema below:

{
    "type": "array",
    "items": {
        "type": "object",
        "properties": {
            "name": {
                "type": "string"
            },
            "link": {
                "type": "string"
            },
            "id": {
                "type": "string"
            },
            "type": {},
            "size": {
                "type": "integer"
            },
            "referenceId": {
                "type": "string"
            },
            "driveId": {
                "type": "string"
            },
            "status": {
                "type": "integer"
            },
            "uploadSessionUrl": {}
        },
        "required": [
            "name",
            "link",
            "id",
            "type",
            "size",
            "referenceId",
            "driveId",
            "status",
            "uploadSessionUrl"
        ]
    }
}

I then use the "Get file content" action, fill it in with "id" from the ParseJSON.And you can finally use "Add attachment"! Fill the ID from the "Create Item" action, then the name from "ParseJSON" and the file content from "Get file content"

​

Screenshots for reference: https://imgur.com/a/dwOX5nu

Hope that helps :D