Sounds like a kind samaritan could write a scanner to find as many RSS feeds as possible which look like RSS/Atom and don't have these content types, then go and patch the hosting software those feeds use to have the correct content types, or ask the webmasters to fix it if they're home-made sites.
As soon as a majority of sites use the correct types, clients can start requiring it for newly added feeds, which in turn will make webmasters make it right if they want their feed to work.
Not even Cloudflares own blog uses those, https://blog.cloudflare.com/rss/, or am I getting a wrong content-type shown in my dev tools? For me it is `application/xml`. So even if `application/rss+xml` were the correct type by an official spec, it's not something to rely on if it's not used commonly.
I just checked Wikipedia and it says Atom's is 'application/atom+xml' (also confirmed in the IANA registry), and RSS's is 'application/rss+xml' (but it's not registered yet, and 'text/xml' is also used widely).
'application/rss+xml' seems to be the best option though in my opinion. The '+xml' in the media type tells (good) parsers to fall back to using an XML parser if they don't understand the 'rss' part, but the 'rss' part provides more accurate information on the content's type for parsers that do understand RSS.
As soon as a majority of sites use the correct types, clients can start requiring it for newly added feeds, which in turn will make webmasters make it right if they want their feed to work.