The Download manager is just a viewing app for seeing all downloads you have done clearing it or forcing it to stop will do nothing(it will just clear the app's data and cache)
You might want to try clearing cache of chrome app since that's the app you used to download the file by going into Settings -> Apps/Installed apps ->Locate Chrome app -> ...
In the folder /data/data/com.android.chrome and its subfolders, would be the place to look. You will need superuser permission to be able to access this folder. In Total Commander start File System Root.
For example /data/data/com.android.chrome/cache/image_cache Has picture files. Actually all the files in there are pics even though they have no extension.