fix(filetype): use :setf instead of nvim_buf_set_option (#20334)

test/functional/lua/filetype_spec.lua

@@ -1,6 +1,7 @@
 local helpers = require('test.functional.helpers')(after_each)
 local exec_lua = helpers.exec_lua
 local eq = helpers.eq
+local meths = helpers.meths
 local clear = helpers.clear
 local pathroot = helpers.pathroot
 local command = helpers.command
@@ -94,3 +95,10 @@ describe('vim.filetype', function()
     ]])
   end)
 end)
+
+describe('filetype.lua', function()
+  it('does not override user autocommands that set filetype #20333', function()
+    clear({args={'--clean', '--cmd', 'autocmd BufRead *.md set filetype=notmarkdown', 'README.md'}})
+    eq('notmarkdown', meths.buf_get_option(0, 'filetype'))
+  end)
+end)