PART files are created whilst downloading anything from the internet. Once the download is successfully completed, this extension will change to reflect the proper suffix of the file name. If you have a file extension part on your system, it simply means you did not manage to download a file properly.
The part file extension is associated with partially downloaded files from internet. Unless the download is finished, the partial file appears as corrupted and usually cannot be accessed by any means. After the download is finished, the partial file is renamed to its original extension.
Files with .part extension cannot be opened!
They are only temporary, partial files created by your download manager. However you can find software that will manage to identify the source and continue or restart the retrieval process.