To convert a text file to HTML I use txt2html that can be installed directly from the Software Manager of Linux Mint.
To convert a text file, open the directory of the file in the terminal.
Right-click on the directory and choose "Open in terminal".
To convert the file to HTML enter the following:
txt2html -extract -8 -infile 'file name' -outf 'output file name'
The '-extract' option specifies NOT to put HTML headers or footers on the result, just the plain HTML text with paragraph tags. Thus the output can be used for inserting into another HTML document.
The '-8' option specifies NOT to convert Latin-1 characters to HTML entities.
txt2html has many options for input and output. To get these listed enter:
txt2html -help