Include filesize information in REST API response for all media types

[tfrommen]

Requesting the /media REST API endpoint, only audio attachments include filesize information in the response data (in media_details->filesize).

I suggest to check for each item if filesize is present, or else fill it in using the actual file. This would be pretty much in line with what is done in ​wp_prepare_attachment_for_js.

Making the information available locally, when creating the response data, is much more performant than having to request the file in one or more follow-up requests.

[tfrommen]

Formatting

[desrosj]

Today is 5.8 feature freeze. Unfortunately this one ran out of time.

Punting to 5.9 as there has been good recent momentum.

[spacedmonkey]

Related: #49412