[WK2] Attachment icons do not update after updating filewrapper

[kcheney1]

896b287

[WK2] Attachment icons do not update after updating filewrapper
https://bugs.webkit.org/show_bug.cgi?id=241026
rdar://86293273

Reviewed by Wenson Hsieh.

In the case of certain iWork file types downloaded from iCloud that require
thumbnails, we don't remove the progress update once the filewrapper is updated
with the complete attachment, which prevents the thumbnail from appearing.

This patch removes the progress attribute once the thumbnail is updated
so we know we can now paint the icon.

No new tests. There's no clear way to test this change. For previous thumbnail bugs
we swizzled the thumbnail generator code to make sure we were generating
thumbnails; however, this bug is a case of the thumbnail being generated but not
displayed. Similarly, we can't compare the attachment size because it is the same for
the attachment with and without the correct thumbnail.

* Source/WebCore/html/HTMLAttachmentElement.cpp:
(WebCore::HTMLAttachmentElement::updateThumbnail):

Canonical link: https://commits.webkit.org/251080@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@294976 268f45cc-cd09-0410-ab3c-d52691b4dbfc

[whsieh]

It seemed a tiny bit odd for QL thumbnailing to have the side effect of removing a DOM attribute, but given that this is basically how the rest of the WKAttachment client SPI works — e.g. -setFileWrapper:, etc. — I don't think it's too weird.

[webkit-early-warning-system]

Committed r294976 (251080@main): https://commits.webkit.org/251080@main

Reviewed commits have been landed. Closing PR #1108 and removing active labels.