zEdit is a text editor that can open many kinds of text files including standard .txt files, as well as software source code files and log files.

Specifically, you can import these files types from mail attachments: asm, bak, c, cpp, css, csv, dat, h, hpp, htm, html, java, js, lisp, lsp, log, m, mak, md, php, pl, py, rss, rtf, sh, sql, swift, tex, text, txt, xib, xml, xpm.

You can have multiple text files open at once.
You can send any files as an email attachment or as the email body.
You can print files.
You can have a file spoken out loud.
You can use Command keys with an external keyboard.