vim - 为什么 `filetype on` 在 vi​​m 中没有检测到文件类型？

Question

我打开了 vim 而没有加载任何东西:

vim -V -u NONE

然后输入一行:

<?xml?>

运行后:

:unlet did_load_filetypes
:filetype on

我检查当前缓冲区的文件类型:

:set ft?

它打印filetype=

---

如果我输入

:unlet did_load_filetypes
:filetype detect

然后 :set ft? 将打印 filetype=xml。

---

在 vim 文档中:

Detail: The ":filetype on" command will load one of these files:                                    
                Amiga       $VIMRUNTIME/filetype.vim                                                
                Mac         $VIMRUNTIME:filetype.vim                                                
                MS-DOS      $VIMRUNTIME\filetype.vim                                                
                RiscOS      Vim:Filetype                                                            
                Unix        $VIMRUNTIME/filetype.vim                                                
                VMS         $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.   

---

:filetype on 和 :filetype detect 有什么区别？
感谢您的帮助。

Answer 1

来自手册:

To detect the file type again:
    :filetype detect
Use this if you started with an empty file and typed text that makes it
possible to detect the file type.