let g:mkdp_command_for_globa not work

**Describe the bug**

I had set `let g:mkdp_command_for_globa=1` but still can't find MarkdownPreview in file .rmd.

My current solution is set

```
au BufNewFile,BufRead *.Rmd,*.rmd,*.Smd,*.smd setf markdown
```
but the auto start feature still can't work in *.rmd, my nvimrc is

```
let g:mkdp_auto_start = 1
let g:mkdp_command_for_global = 1
let g:mkdp_echo_preview_url = 1
let g:mkdp_open_to_the_world = 1
```