Idea: store filesize attachment meta value on upload.

[goldenapples]

There are a few places in core, like wp_prepare_attachment_for_js, that do a filesystem stat check for attachments to get the filesize in bytes:

https://github.com/WordPress/WordPress/blob/master/wp-includes/media.php#L3969-L3975

This can be short-circuited if the "filesize" property is set in the attachment meta array.

Setting this property seems as though it would eliminate the need for several AWS S3 requests which happen as part of the query-attachments ajax action. Is it feasible to set it on uploading attachments?