How about S3-like storage?

[janboddez]

Depends on the specific plugin in use, probably. I mean, if the images aren't stored locally, we can't just upload them, I suppose.

[janboddez]

Affected sites (this isn't a bug per se, by the way) could filter attachment_url_to_postid. Or we could get the IDs from Gutenberg's Image blocks.

[janboddez]

So, to be able to fetch image IDs for in-post content, we'd have to filter attachment_url_to_postid.

But that's just one part of the puzzle. Given these IDs (or that of a featured image, or attached images, for which we already know the ID), we still need to re-convert them to a (local) file.

Assuming the local file (still) exists, it's a matter of hooking into overriding both wp_get_attachment_image_src() and wp_get_attachment_url(). Or, we could add a "shortcut" to Image_Handler::upload_image(), to set $file_path without calling these functions at all.

If the local file does not exist, I guess we could wp_remote_get() it (or similar), and still send it along.

Or we could just not support any of that (although merely adding the hooks doesn't hurt).

[janboddez]

So, to be able to fetch image IDs for in-post content, we'd have to filter attachment_url_to_postid.

Unless they're served through a reverse proxy handling the default WordPress (e.g., wp-content/uploads*) URLs. So this is the "less critical" part of the whole operation.

The other part is downloading the file data in order to then upload it.

We use file_get_contents( $file_path ) were $file_path is assumed to be a local path on the server. But it could be s3://bucket_name/<etc.> too, depending on what plugin was in use. And I'm not sure if that would work (doubt it).

If it's not a file PHP can read (we actually check is_file( $file_path )), nothing happens.

We could "prefilter" this path, download the file to a temp folder (that we prevent access to), set $file_path accordingly, then upload it after it is sent successfully.

[janboddez]

Currently using S3 Uploads, and the paths (i.e., the basedir returned by wp_upload_dir()) do indeed look like s3://bucket_name/<etc.>, and I can upload these images OK. I.e., PHP seems able to read them just fine using nothing but file_get_contents( $file_path ).