It's not a custom field, it's a system field. It's global, so you can only have one.
When you're adding fields to a *screen* it will be on the list. Once it's added, you need to make sure your users have the rights to add/update attachments too, or it will be hidden from them (in the permission scheme)
Are you looking for an attachment custom field type? There are rumors that it is possible to create a plugin to do this or you can add your voice to the votes for Atlassian to add this to JIRA as a feature:
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