PART files are files that have only been partially downloaded by a download application. This happens when a user's download manager stores a portion of the file that you are downloading on your hard drive as the download is occurring.
Certain download managers will break large downloads up into smaller downloads, giving each portion of the download the .part extension. The download manager will then combine all of the .part files into the complete file after the download has finished. At this time, the combined .part files will be renamed with the proper file extension.
Popular programs that commonly use the .part file extension include eMule and Go!Zilla. While .part files cannot be opened themselves, there are programs that can help you continue the download and retrieval process if a download is interrupted.
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If you have additional information about the PART file format or software that uses files with the PART suffix, please do get in touch - we would love hearing from you.