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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Use *const AtomicUsize rather than *mut AtomicUsize in WorkQueue.
Atomics use internal mutability, so this *mut is at best confusing.
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Account for the font of empty input fields
The font of empty fragments is not accounted for in layout. This behavior is incorrect when part of an input field.
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Silent some warnings
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Update Blob::{new, new_inherited} to take Strings
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