jsx:"preserve" + allowJs converts js to jsx

[testerez]

When transpiling js files with the new allowJs param and jsx:"preserve", all js files are output with jsx extention. Here is my tsconfig.json:

{
    "compilerOptions": {
        "module": "es2015",
        "allowJs": true,
        "target": "es2015",
        "jsx": "preserve"
    }
}

[billti]

Duplicate of #6948 . (And fixed with #6949)