I run several installations and they are all having the same problem.
Images are disappearing in subdomains after upgrading. I can't say when it started, because I wasn't paying attention.
My blogs go back to days when media files were uploaded to wp-content/uploads/YYYY/MM where YYYY is the year and MM is the month.
I just recently noticed that all of my subdomains have posts with missing images. The images are still in wp-content/uploads/... but they are not being looked for there.
Instead, if I inspect the <img> tag, src attribute looks like:
I created a wp-content/blogs.dir/n/files subdirectory, gave it all of the right ownership and permissions. It still won't show up in the post. (n is the blog number from wp_blogs)
I tried changing the subdirectory structure to wp-content/blogs/n/myimage.jpg and it still won't show the picture.
What is the mechanism that takes a URL like subdomain.example.com/files/myimage.jpg and makes it look in blogs.dir/n/files/myimage.jpg or blogs/n/files/myimage.jpg?
Whatever it is, it isn't working on any of my domains.