Webmail - settings for attachments

[#1]

Just playing about and under Preferences/Composing Messages there are three options for attachments
Full RFC 2231 (Thunderbird)
RFC 2047/2231 (MS Outlook)
RFC 2047 (other)
What is the significance of this and why is Thunderbird the default  

[#2]

The codes are the different standards (MIME types) for email attachments and the programs that use them respectively. I'd guess Thunderbird is the default because it uses the most recent specification listed.
At the end of the day though it should work fine whichever you choose, I've never had a problem sending emails with attachments between Thunderbird and Outlook.

[#3]

I don't think one should say "Thunderbird" is the default - "RFC 2231", the latest standard, is the default; "Thunderbird" is listed as an application that understands and creates RFC 2231 attachment headers. Since the RFCs are backwards compatible it will also understand RFC 2047 attachment headers but probably not downgrade any response to RFC 2047 standard.
Presumably MS Outlook will (optionally) downgrade; don't know how Roundcube handles this. I suspect it's an issue mainly for non-ASCII character sets and non-English languages.