The AttachFile action enables a page to have multiple attached files. Since file uploads could be abused for DoS (Denial of Service) attacks, AttachFile is an action that may be disabled by the wiki administrator (by adding actions_excluded = ['AttachFile'] to wikiconfig).
This is all you usually need to do for configuration.
How attachments are handled
Attachments are stored "privately" in the data/pages/ directory (with paths like data/pages/<pagename>/attachments/<filename>) and can only be retrieved by using the AttachFile action (via URLs like http://example.org/mywiki/<SomePage>?action=AttachFile&do=get&target=filename.ext).
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The direct serving method (deprecated since long) was finally removed in moin 1.7.0. If you have used it until now (attachments = {...}), please copy your attachments into your data directory (and make sure they arrive at the right location there).