Prefer message timestamp over the last-modified header for asset file dates

[ritiek]

Fixes #1320.

With this PR, date-modified timestamp for attachments now prefers the timestamp of the message in which the attachment was originally shared on Discord:

Performing export of the same channel as originally shared in #1320:

$ DISCORD_TOKEN="xyz" dotnet run --project DiscordChatExporter.Cli/DiscordChatExporter.Cli.csproj export --channel 869238366951079956 --output . --media --reuse-media --markdown false --format Json --after 1296272363444375632 --before 1312437546873131059

$ stat -c '%y' "Tyrrrz: Open Source - 📦 Projects - youtube-downloader [869238366951079956] (2024-10-17 to 2024-11-30).json_Files"/image*.png
2024-11-01 02:30:27.462000000 +0530
2024-11-17 15:37:14.379000000 +0530
2024-11-01 03:45:28.735000000 +0530
2024-11-01 03:31:24.518000000 +0530
2024-11-22 14:50:38.695000000 +0530
2024-10-29 15:21:46.986000000 +0530
2024-11-01 03:45:28.735000000 +0530
2024-11-03 20:56:30.421000000 +0530
2024-11-01 03:48:04.651000000 +0530
2024-11-01 03:37:37.762000000 +0530
2024-11-01 03:31:24.518000000 +0530
2024-11-01 03:37:37.762000000 +0530
2024-11-01 03:28:05.389000000 +0530
2024-11-01 03:45:28.735000000 +0530

Not sure if this is the best approach to implement this, but it seems to work well enough for my current use-case.

Copilot reviewed 3 out of 3 changed files in this pull request and generated no suggestions.

Comments skipped due to low confidence (1)

DiscordChatExporter.Core/Exporting/ExportAssetDownloader.cs:54

• The indentation of the 'if (timestamp is null)' block is incorrect. It should be aligned with the rest of the code.

{