Discord's account data export provides a messages folder, containing subfolders that have a json for each channel/user ID.
Example file: ./messages/c777777777777777777/messages.json
[
{"ID": 469641859543728149, "Timestamp": "2018-07-19 23:09:04", "Contents": "ayy its your boi", "Attachments": ""}
]
It would be great for Selfie to have an easy way to import either the folder or the export.zip to import a large quantity of messages.