How to change output format

[caroott]

I'm currently trying to encode a .ppm file lossless and getting the same file (.ppm) back when decoding. Somehow i always end up with a .png file in the end and have no idea how to change that.

I'm currently doing the encoding and decoding in F#.
This is an example of how the code looks like. I also tried changing the parameters, but the output fileformat didn't change. The picture i got in the last step was always a PNG, while the input was a PPM. I tried different PPM pictures (including the samples from the testfile).

let image = File.OpenRead @"filepath"
let enc = J2kImage.ToBytes(J2kImage.CreateEncodableSource(image2))
File.WriteAllBytes(@"filepath2", enc)

let dec = J2kImage.FromFile(@"filepath2")
let dec2 = dec.As()
dec2.Save(@"filepath3")

Thanks in advance for any help.