When creating or updating a chat message, uploads attached to the
message are filtered so as to only keep uploads created by the message
creator using `Upload.user`. This field, however, only points to the
_original_ user that created the upload, but since uploads are
de-duplicated, other users might have also uploaded the file. This PR
fixes this by looking at the `UserUpload`s instead (as suggested by
@SamSaffron).
Reported here: https://meta.discourse.org/t/chat-upload-bug/379253