Jason Song 7a8e34b255 Deal with markdown template without metadata (#21639) (#21654)
Backport #21639 .

Fixed #21636.

Related to #20987.

A markdown template without metadata should not be treated as an invalid
template.

And this PR fixed another bug that non-template files(neither .md nor
.yaml) are treated as yaml files.

<img width="504" alt="image"

src="https://user-images.githubusercontent.com/9418365/198968668-40082fa1-4f25-4d3e-9b73-1dbf6d1a7521.png">
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