fix: add image/jpg to MIME type extension lookup

Although image/jpg is not a registered MIME type (RFC 2046 specifies
image/jpeg), several external providers return image/jpg as the
Content-Type for JPEG images:

- TMDb API (thumbnail/episode images)
- Schedules Direct EPG
- Various other metadata providers

Without this mapping, Jellyfin throws ArgumentException:
'Unable to determine image file extension from mime type image/jpg'

This causes library scans to fail when saving episode thumbnails
and other images from these providers, leading to repeated scan
failures and cancelled library scans.

PR #7052 previously added this but it was lost during the migration
to FrozenDictionary. Issue #13568 reports the same bug in 10.10+.

Fixes: jellyfin/jellyfin#13568
Related: jellyfin/jellyfin#7050, jellyfin/jellyfin#7052

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