diff --git a/coretool.js b/coretool.js
index 1f0d0ff..a8f8227 100644
--- a/coretool.js
+++ b/coretool.js
@@ -389,7 +389,26 @@ module.exports = {
   },
 
   segment_number (sizeBytes = '') { // segment a number with a space between each 3 digits
-    return (sizeBytes).toString().replace(/\B(?=(\d{3})+(?!\d))/g, ' ')
+    let segmented = (sizeBytes).toString().replace(/\B(?=(\d{3})+(?!\d))/g, ' ')
+    const parts = segmented.split(' ')
+    parts[parts.length - 3] += 'm'
+    parts[parts.length - 2] += 'k'
+    return parts.join(' ') + 'b'
   },
 
+  get_file_ext (filename = '') {
+    const parts = filename.split('.')
+    // 注意，'abc'.split('.') === ['abc']
+    let ext = ''
+    if (parts.length >= 2) {
+      ext = parts.pop().toLowerCase()
+    }
+    return ext
+
+    // 如果用 path.extname:
+    // if (/^\./.test(filename)) filename = `added$filename` // path.extname('.abc') 结果为 ''，所以要添加前缀
+    // return path.extname(filename).toLowerCase() // openAi*Ext 是包含 . 的，所以用 path.extname
+
+  }
+
 }