Now my default file type of Visual Studio Code is Plain Text.
If I save a new file with name like a.in, it will save as a.in.txt. I have to change the dropdown to All Files.
I find the same problems in github: 1, 2. However, they seem not solve my problem.
There is a way to change the default extension (but not to All Files)
Add the following line to your settings.json
"files.defaultLanguage": "<language>",
Replace <language> with the language of your choice.
Now, whenever you make a new a file, the default file language will be <language>.
A special value for <language> is ${activeEditorLanguage} which is the language of the file last opened (useful if, say, you copy a piece of code from one file to save as another).
Unfortunately, this does not fully answer the question, but provides a partial solution.