Import content from a WordPress export file. Content that already exists is added as a post revision. Also lets you specify a default parent post.
The importer has a couple of filters to allow you to completely enable/block certain features:
import_allow_create_users: return false if you only want to allow mapping to existing users
import_allow_fetch_attachments: return false if you do not wish to allow importing and downloading of attachments
import_attachment_size_limit: return an integer value for the maximum file size in bytes to save (default is 0, which is unlimited)
There are also a few actions available to hook into:
import_start: occurs after the export file has been uploaded and author import settings have been chosen
import_end: called after the last output from the importer