JSON lines Mime type
As per https://github.com/wardi/jsonlines/issues/9, there is no official mime type, but a convention of application/x-ndjson. Or application/x-jsonlines. Note that this issue has been sitting there since 2015, which might suggest that jsonlines is not exactly catching fire.
I have the server return text/plain; charset=utf8 for JSON Lines files. As @bmargulies says, application/x-ndjson has limited support. At least with text/plain users can see the data in their browser (rather than being shown a download/save as dialog). Setting the charset explicitly prevents random "corruption" by (un)helpful user agents.