Add an SQLite content type to uploaded exports

There are two MIME types for SQLite: application/x-sqlite and application/vnd.sqlite3. The X- prefix has been deprecated by RFC 6648, so we should prefer vnd.sqlite3 which has been assigned by IANA. Neither of the two content types were detected properly by my web browser, so this isn't much more than a nit, but setting it would allow someone to at least assign the type themselves when saving.