The content-type is being sent as text/csv instead of text/plain resulting in some browsers downloading a plain.txt as plain.csv. You can see this in action below...
GET /o/response.txt HTTP/2
Host: linx.server
User-Agent: curl/8.7.1
Accept: /
Request completely sent off
< HTTP/2 200
< date: Sat, 29 Mar 2025 17:23:21 GMT
< content-type: text/csv
The content-type is being sent as text/csv instead of text/plain resulting in some browsers downloading a plain.txt as plain.csv. You can see this in action below...