It would be great to have built-in support for various file-sharing services. Or at least, provide a plugin API so 3rd-party contributors could develop such. Self-hosted solutions such as Nextcloud or cloud options like Google Drive, OneDrive, etc. are commonly used with business today and having to manually upload to the drive, then copy the file-share link, instead of having the client handle such automatically is a hassle.
Thunderbird has included support for such for quite some time.