Fix get_attachment_files when files are in subdirs

[joehoyle]

In cases when a file is not directly in wp_upload_dir()['basedir'], Plugin::get_attachment_files will fail to include sub directories that the file is in.

$meta['sizes'] and co should always be relative to the directory that the main file is in.

[hm-backport]

The backport to v3-branch failed:

Not Found

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport v3-branch
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick 8ffd577352d4a0979102d0db2c63476828a0cd22 b749a8b6cf57e63720a52b3d2a6ad0b658db55cf
# Create a new branch with these backported commits.
git checkout -b backport-712-to-v3-branch
# Push it to GitHub.
git push --set-upstream origin backport-712-to-v3-branch
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is v3-branch and the compare/head branch is backport-712-to-v3-branch.