[BUG] featured-image only works on index.md file

[DilemmaRabbit]

Hello, I'm trying to add featured-image for my post.
My content structure is like this

• content
  • blogs
    • A.md
    • B.md
    • C.md
    • featured-image.jpg

I want to use featured-image.jpg for A,B,C

  - name: "featured-image"
    src: "featured-image.jpg"

This is what I add in my .md file.
But this is not work.

I notice that the file named with "index.md" will work perfectly(like example post).
Can I keep my old file name instead of index.md with featured-image?