The story so far - back in the early, very early days of YouTube - so 2007 or so, when 'downloading' a video for offline use meant grabbing the 240p or 360p .flv (Flash Video) files, I archived hundreds of my favourite clips and music videos. Maybe you did too. And then, over time, it became possible to save higher resolution clips, 480p and then 720p and beyond. all in MP4, which was much easier to deal with on almost any device.  So .flv files were forgotten by most people, including me - I somehow lost all the files and never missed them. And then, in 2016, along comes a working FLV player for Windows Phone. Eh, what?

From the app description:

Client for FLV - the best player for FLV video files. With this app you can watch your favorite movies and videos in FLV format is really convenient. Forget about the problems that are associated with video playback! Client for FLV present you the perfect viewing video files and only the most positive emotions. Minimalistic interface makes it very comfortable to play media files, stylish design pleases the eyes, the minimum requirements to resources let you watch videos without interference.

• Convenient playback of video files in a format FLV
• Creating a list of recently viewed videos
• Search for videos by the app
• The ability to share video
• The ability to watch videos from OneDrive
• The ability to open video via the file picker.

Here's Client for FLV in action, with notes from me:

When started, there's no obvious way to get your FLV videos into the app, but a little digging reveals that it's looking on devices in the \Videos folder, so I did some downloading and dragging and dropping across from a Windows PC. Windows 10 complains, like you knew it would, thinking that you won't be able to play the files. I chose 'just copy' and all was well.

And lo and behold, one 'refresh' later and there were my new FLV video files. Here's the About screen, it does seem to have got an update this year, at least, and works well with Windows Phone 8.1 and Windows 10 Mobile.

Playback is surprisingly smooth and you can scrub along the timeline - which is some achievement for the horrifically non-linear format that is Flash Video!

Of course, the source material is usually 240p or similar, so don't expect the earth. But still, it works and if you have an old archive of .flv files, now there's a reason to load them onto your phone!

At the time of writing, this is free, but the offer won't last much longer (it's part of a myAppFree deal), so grab it now etc.