Pass in base64 encoded images as input (invalid PNG signature error)

**Describe the bug**
I'm receiving an error when passing in a base64 encoded image as the input, here are what I think are the relevant parts of the logs:

`Invalid PNG signature `
`Consider increasing the value for the 'analyzeduration' and 'probesize' options`
`Input #1, image2, from 'image.png':`
`Error while decoding stream #1:0: Invalid data found when processing input`
`Cannot determine format of input stream 1:0 after EOF`
`Error marking filters as finished`
`Conversion failed!`

My code is this:

fetches the video file:
`ffmpeg.FS('writeFile', name, await fetchFile(files[0]))`

this is the base64 data uri:
`await ffmpeg.FS('writeFile', 'image.png', payload.elements[0].img)`

my complex filter is a variable string, but I've eliminated this as being related to the cause of the issue:
`await ffmpeg.run('-i', name, '-i', 'image.png', '-filter_complex', `${complexFilter}`, '-map', '[out_v]' , '-pix_fmt', 'yuv420p', 'output.mp4')`

I've tried passing it in as a base64 data uri, as just the raw base64 data, as a blob, I can't seem to work out the correct syntax. I've looked through the example scripts, but they retrieve from a local directory rather than encoded images. I'd appreciate it if anyone could help me out with the syntax here.

If it's helpful, here's a link to the base64 string that's inside of payload.elements[0].img:
https://pastebin.com/EZNewbWX

[This site](https://codebeautify.org/base64-to-image-converter) can be used to decode it and show an image, which verifies the base64 data is valid.