File extension .PART

As the name says file extension part means literally: Partially downloaded file and it is mainly used when downloading unfinished downloaded files from internet.
It is also used by Firefox browser to store a partially downloaded file and after downloading the complete file, it is renamed into the right original file type.
The file extension part is used in case of large amounts of information, to avoid, in case of connection break, that the data already downloaded go lost.
It is the most used among P2P users.
In this case networks files are not stored on one central server, but shared among computers, which divide and store small files of a complete movie or other.

P2P downloading is never 100% safe, on the contrary, one should be aware that these files can often contain Trojans and viruses. (Besides of course the legal consequences of downloading illegal content)
Also in this case, if Windows is unable to open the file extension part you may have registry related errors or may not have the appropriate program installed.
And also in this case it could be very useful to run a scan of your registry.