Auto merge of #11757 - izgzhen:file-acccept-mime-filter, r=Manishearth

Implement filter for file-type input's accept attribute

Now the two sides are pasted together with the new version of `mime_guess` landed.

I tested this thing locally with stuff like this:

```html
<input id="input_file" type="file" accept=".doc,.docx,.xml,application/msword,application/vnd.openxmlformats-officedocument.wordprocessingml.document"></input>
<a onclick="open_file();">Click</a>

<script type="text/javascript">
    function open_file() {
        console.log("Open file");
        document.getElementById("input_file").click();
    }
</script>
```

Will WPT be able to handle this automatically?

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