I'm not exactly sure I understand what you mean by "way to extract the single files".
aMule's temp folder (see the preferences for its location) contains all the incomplete files. The *.part file contains the actual file data for a given download and the *.part.met file contains information about which chunks have already been downloaded of that file, etc.
Once all the data for a given file has been downloaded and verified, the corresponding .part file is simply renamed and moved into your incoming folder. So the .part file is really the (incomplete) final file. So you can simply open a .part file in for instance VLC to preview it (that's exactly what aMule does when you choose the "preview" option). You can also open a .part file in other apps. For instance, if it is a zip file, you might be able to extract parts of the archive that have already been downloaded.