Thanks for replying.
Right after posting my question, I thought to check the MediaInfo, and I copied and pasted it in response to Kees' reply. I initially found that it is an FLV_27 file by checking the file's Properties and saw .flv_27, and the name of the file itself has .flv_27 at the end of it. The MediaInfo confirmed it.
The downloaded file is a documentary film, and I know it's a risk to download from file sharing sites. When you say there are sites that install infected decoders, do you mean free file converter software? I'll try the VLC Media Player you recommended and see if it will open the file and convert it.