In the .htaccess file of a multisite installation, you’ll find this little bit of code:
# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]
The effect of this code is to redirect requests that have “files” in the name to the ms-files.php. As you may have guessed, ms-files.php is the multi-site file handler.
Files in multi-site mode are stored in the /wp-content/blogs.dir structure. Inside that directory, you will find subdirectories labeled with numbers (1, 2, 3, etc.). Each number corresponds to the ID number of the individual site in the multi-site installation. Each one of these directories holds all the uploaded files for that installation. The ms-files.php file handles a bunch of caching parameters and then sends the file off to the browser upon request.
So the resulting URLs always look like http://example.com/files/2010/whatever.jpg and so forth. The “files” name is therefore special and reserved and cannot be used as a post slug and such.
When you export a site from somewhere and then import it into your new multi-site system, if you also choose to import the attachments, then you’ll have a minor problem. The attachments will be imported into this new files structure. However, the links from an older system are still referring to the old /wp-content/uploads/ directory structure that non-multi-site installs use. Therefore, you will need to go back through your posts and fix all these links and references to the attachments. I use the Search Regex plugin for this purpose, it works well enough and has some powerful search and replace capabilities.