Some people have slow network connections. If files begin to appear on the list, it would take these people an inordinate amount of time to download their mail. If you have a file to distribute, you should post it to your website and send the URL to the list. HTML is the language of the World Wide Web. If your mail program allows you to change the colour, font, size or any other aspect of the text you are writing, you are probably posting to the list using this document markup standard.
Plain text is just that.
HTML more than doubles the size of your post, increases the time it takes to download messages in the group, and makes it difficult for people with non-HTML enabled mail programs to read. Consult the documentation provided with your email package to disable this feature. Flames are heated discussions which are usually personal and almost certainly off-topic. They can get big very quickly, and that's called a flamewar. If you see a message on the list which you personally object to, please exercise restraint.