There are two storage/retrieval models for file attachments:
Attachments are stored "privately" and can only be retrieved via a CGI GET (via URLs like http://myorg.org/mywiki/<SomePage>?action=AttachFile&do=get&target=filename.ext).
Attachments are stored into a directory directly accessible by the web server, and can thus be served directly by the webserver, without any invocation of MoinMoin (leading to URLs like http://myorg.org/mywikiattach/<Somepage>/attachments/filename.ext).
The first option is the default; attachments are stored in the "...mywiki/data/pages/" directory, with paths like "...mywiki/data/pages/<pagename>/attachments/<filename>".
The MoinMoin attachments configuration option allows you to move the directory structure used to store attachments to another location. Unless you have a reason for doing so, there is no need to use a different location. Using a different location may be more work and more risk, as all the existing attachments must be copied to the new location. The following instructions are for Apache servers and assume you intend to leave the attachment files in their existing location and your original installation used the name "mywiki".