Fix check for restricted attachments

[ttys0dev]

Looks like this was broken in #4352.

[albertisfu]

Thanks @ttys0dev I left one comment in code. Beside that I just want to confirm that with this tweak restricted attachments will be omitted during merging.

Before the tweak:

After:

On PACER, these attachments are shown even though they're sealed.

So I was wondering if this behavior is correct, @mlissner?

[mlissner]

Yeah, I don't think that's right. I think if it's sealed, we still want to get the metadata for the case, including the short_description that says, *Restricted*. If we want to get really elegant, we have an issue somewhere to detect sealed stuff and actually flip the sealed boolean in the DB to True, but it hasn't gotten prioritized yet.