vim turns on file type detection, that is, "filetype on", and turns off vi compatibility, "set nocompatible". According to the documentation:
Detail: The ":filetype on" command will load one of these files:
Mac $VIMRUNTIME:filetype.vim
Unix $VIMRUNTIME/filetype.vim
This file is a Vim script that defines autocommands for the
BufNewFile and BufRead events. If the file type is not found by the
name, the file $VIMRUNTIME/scripts.vim is used to detect it from the
contents of the file.
There are the following entries about markdown in filetype.vim:
" Markdown
au BufNewFile,BufRead *.markdown,*.mdown,*.mkd,*.mkdn,README.md setf markdown
Explanation README.md will be treated as a markdown file.
But after I opened the markdown file, it was displayed as a vimwiki file (the plug-in vimwiki installed for me), as shown below: 
What is the reason for this?
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
The easiest solution to this is to change the default setting of the g:vimwiki_ext2syntax variable in your .vimrc (or Janus could make this the default) as follows:
let g:vimwiki_ext2syntax = {}
The default value of g:vimwikiext2syntax = {'.md': 'markdown'}. This setting tells vimwiki to recognize files with a ".md" extension, overriding previous filetype associations. See ":help vimwikiext2syntax".
vimwiki/markdown conflict
Thanks for the invitation... I opened README.md and got the correct result... So my guess is that vimwiki has overwritten vim's default configuration...
Try it in
g:vimwiki_list里面去掉.mdfile ..?