A PART file is a partially downloaded file used for downloads that are in progress or have been stopped. Some PART files can be resumed later using the same program that started the download. PART files are typically used by Mozilla Firefox and file transfer programs like Go!Zilla and eMule.
When downloading a file, Firefox renames the file and appends the .part extension. For example, example.mov would become ZT5KR1EL.mov.part. Once the download is complete, Firefox removes the .part extension.
You may also encounter PART files when using Skype on a Mac. Overall, PART files allow downloading of large internet files in chunks. However, interrupted downloads may result in corrupt files.
A .PART file cannot be opened directly. Once the download is complete and .part extension removed, the file can be opened normally. If interrupted, the file may be corrupt and unable to open.
When applications like Firefox, Free Download Manager and JDownloader start a download, they add .PART to the filename. For example, file.mp4 would become file.mp4.part. After completing the download, the program restores the original filename.
The .part extension results from an interrupted download. The browser or program like eMule stores part of the file while downloading. This allows resume of the download if interrupted. The .part file tracks download progress.
Mozilla Firefox
Google Chrome
Internet Download Manager
Autodesk Fusion 360