When bulk import finds an already existing post, it should delete existing files before importing new ones

Description

Currently, when importing a Mattermost export file, if the importer finds a post that matches the post timestamp that its trying to import for the same channel, it will consider that both posts are the same, and it will append the new post attachments to the already imported ones.

This is not an idempotent behavior, so we should modify the logic to delete previously existing files before importing new ones.

QA Test Steps

From JE on the PR: Probably some dev help needed here to build an example import dump

Mana

None

Assignee

Mattermost Community

QA Assignee

Saturnino Abril

Reporter

Miguel de la Cruz

Epic Link

None

Fix versions

Mattermost Team

Platform

Sprint

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Labels

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QA Testing Areas

CLI

GitHub Issue

None

Components

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