feat(icon): add opt-in filetype detection via file contents

Problem: files without standard extensions (e.g. scripts with
shebangs, Makefile, Dockerfile) got incorrect or default icons since
icon providers match by filename or extension only.

Solution: add use_slow_filetype_detection option to the icon column
config. When enabled, reads the first 16 lines of each file and
passes them to vim.filetype.match for content-based detection. The
detected filetype is forwarded to mini.icons or nvim-web-devicons as
a trailing parameter, preserving backwards compatibility with
existing icon provider implementations.

Based on: stevearc/oil.nvim#618

doc/oil.txt

@@ -438,6 +438,10 @@ icon                                                                 *column-ico
       {directory}    `string` Icon for directories
       {add_padding}  `boolean` Set to false to remove the extra whitespace after
                      the icon
+      {use_slow_filetype_detection} `boolean` Set to true to detect filetypes
+                     by reading the first lines of each file (e.g. shebangs).
+                     This improves icon accuracy for extensionless scripts but
+                     performs synchronous I/O per file per render.
 
 size                                                                 *column-size*
     Adapters: files, ssh, s3

lua/oil/columns.lua

@@ -153,7 +153,7 @@ end
 local icon_provider = util.get_icon_provider()
 if icon_provider then
   M.register("icon", {
-    render = function(entry, conf)
+    render = function(entry, conf, bufnr)
       local field_type = entry[FIELD_TYPE]
       local name = entry[FIELD_NAME]
       local meta = entry[FIELD_META]
@@ -168,7 +168,20 @@ if icon_provider then
       if meta and meta.display_name then
         name = meta.display_name
       end
-      local icon, hl = icon_provider(field_type, name, conf)
+
+      local ft = nil
+      if conf and conf.use_slow_filetype_detection and field_type == "file" then
+        local bufname = vim.api.nvim_buf_get_name(bufnr)
+        local _, path = util.parse_url(bufname)
+        if path then
+          local lines = vim.fn.readfile(path .. name, "", 16)
+          if lines and #lines > 0 then
+            ft = vim.filetype.match({ filename = name, contents = lines })
+          end
+        end
+      end
+
+      local icon, hl = icon_provider(field_type, name, conf, ft)
       if not conf or conf.add_padding ~= false then
         icon = icon .. " "
       end

lua/oil/util.lua

@@ -8,7 +8,7 @@ local FIELD_NAME = constants.FIELD_NAME
 local FIELD_TYPE = constants.FIELD_TYPE
 local FIELD_META = constants.FIELD_META
 
----@alias oil.IconProvider fun(type: string, name: string, conf: table?): (icon: string, hl: string)
+---@alias oil.IconProvider fun(type: string, name: string, conf: table?, ft: string?): (icon: string, hl: string)
 
 ---@param url string
 ---@return nil|string
@@ -934,7 +934,10 @@ M.get_icon_provider = function()
   local _, mini_icons = pcall(require, "mini.icons")
   ---@diagnostic disable-next-line: undefined-field
   if _G.MiniIcons then -- `_G.MiniIcons` is a better check to see if the module is setup
-    return function(type, name)
+    return function(type, name, conf, ft)
+      if ft then
+        return mini_icons.get("filetype", ft)
+      end
       return mini_icons.get(type == "directory" and "directory" or "file", name)
     end
   end
@@ -942,10 +945,16 @@ M.get_icon_provider = function()
   -- fallback to `nvim-web-devicons`
   local has_devicons, devicons = pcall(require, "nvim-web-devicons")
   if has_devicons then
-    return function(type, name, conf)
+    return function(type, name, conf, ft)
       if type == "directory" then
         return conf and conf.directory or "", "OilDirIcon"
       else
+        if ft then
+          local ft_icon, ft_hl = devicons.get_icon_by_filetype(ft)
+          if ft_icon and ft_icon ~= "" then
+            return ft_icon, ft_hl
+          end
+        end
         local icon, hl = devicons.get_icon(name)
         icon = icon or (conf and conf.default_file or "")
         return icon, hl

scripts/generate.py

@@ -154,6 +154,11 @@ COL_DEFS = [
                 "boolean",
                 "Set to false to remove the extra whitespace after the icon",
             ),
+            LuaParam(
+                "use_slow_filetype_detection",
+                "boolean",
+                "Set to true to detect filetypes by reading the first lines of each file (e.g. shebangs).",
+            ),
         ],
     ),
     ColumnDef("size", "files, ssh, s3", False, True, "The size of the file", UNIVERSAL + []),