`\middle|` doesn't have MathML attribute `stretchy="true"`

Is your feature request related to a problem? Please describe. Convert \left\{x\middle|x>\sqrt{\frac1n}\right\} to MathML by MathJax, and render in Firefox, the vertical bar won't stretch.

Describe the solution you'd like Add MathML attribute stretchy="true" to \middle| Convert \left\{x\mmlToken{mo}[stretchy="true"]{|}x>\sqrt{\frac1n}\right\} to MathML by MathJax, and render in Firefox, the vertical bar will stretch. Or add MathML attribute form="prefix" as temml.