I don't know of digital-mode software that has the capability to open a file, but that doesn't mean that it doesn't exist. However, there is a handy work-around to your problem: a virtual audio cable.
A "virtual audio cable" is a piece of software that registers itself with the operating system as being both an audio input device and an audio output device. Of course the audio input and output devices are connected together in software, which is why it's called a virtual cable.
So you install virtual audio cable software, and then you connect it to both your software that plays back the recording and your digital-mode software, which interprets the audio as SSTV. Then you press "play" in your audio playback software, and your digital-mode software should be able to decode the SSTV for you.
There is Windows software called Virtual Audio Cable, which is not free to use, but there are other software packages that can be found with a web search that do the same thing, and many of those are free.