PART, Partially Downloaded File (.part)
The PART Misc file format
A file with a PART file extension name is basically a partial download, a small piece of a larger download. There are many different programs that use a PART file to handle downloads, especially when they are unusually large in size. The download manager on your computer will chop up the larger download into smaller pieces. Each of these smaller PART files will be stored on the hard drive of your computer until the entire download has been completed. Upon completion of the download, the download manager will combine all of the separate PART files and complete the download process for the entire file.
Technical details of PART files
When the download manager on your computer completes the downloading of the final PART file, it will automatically combine all of the smaller download files. It will then rename the complete and combined file with the proper extension name. In most cases, a PART file is not intended to be opened since it is only one small piece of the larger download. If you have trouble, you should first try to re-download the original file and see if your computer can download the remaining pieces of the download. There are several different types of file transfer programs that also use the PART functionality. This includes eMule, Go!Zilla and eDonkey.