A DOWNLOAD file is a partially downloaded file from internet that is still in the process of getting downloaded. When you download a file from internet using Apple Safari browser, it may take some time for the download to complete. This partially downloaded file is saved to Downloads folder. If the download is interrupted for any reason, the file can be double-clicked for trying to resume it.
DOWNLOAD File Format - More Information
DOWNLOAD files are saved to disc in text as well as binary file formats. It does not contain the entire file that is in process of download. Once the download is complete, the .download file extension is replaced with the original extension of the file that is being downloaded. For example, if you are downloading a video from internet in MP4 file format, the file will be downloaded ands saved with .download extension. Once the downloaded is complete, the file extension will be changed to .mp4.
DOWNLAOD files are similar to .CRDOWNLOAD files used by the Google Chrome and Microsoft Edge.