Restore the filename argument to FormData.append(name, blob)

[foolip]

Context: https://bugzilla.mozilla.org/show_bug.cgi?id=1162658

I would propose this IDL:

void append(USVString name, Blob value, optional USVString filename);
void append(USVString name, USVString value);

(This is what Blink does.)

[foolip]

@annevk, thoughts? I stumbled across this again now because the Blob is effectively nullable in Blink now, which I'd like to fix, but it matters what happens to this issue. As the spec is written append('', null, '') would stringify the value to "null" I think, but if I make the change I want in Blink it'd instead throw a TypeError as three arguments makes it try the Blob variant. Maybe this is just another bug on Blink's part, but in any case it'd be nice to know how this will end up in the spec.

[foolip]

Thanks, the fix looks good, I'll try to implement that in Blink when I have some spare time.