Could you show the output of uplink meta get sj://path/to/file/index.html?
It seems like linksharing might not be detecting the right content type. It tries to auto-detect based on the extension. If Content-Type is set in metadata (or content-type in lowercase put by S3 clients), it will use that instead of guessing.
We have a fix on the way around these content types to auto-detect, but one way to ensure the file gets the right type is to set --metadata '{"Content-Type":"text/html"} when uploading the file with uplink.
If using S3, you can set content type on an existing file by aws s3 cp with the same source and destination, e.g. aws s3 cp s3://path/to/file s3://path/to/file --content-type text/html. The same flag can be used when uploading a new file as well.