Detecting the format works for a lot of image formats but that is not possible for the TGA format. This format has no marker at the beginning that we can use to identify it (see https://en.wikipedia.org/wiki/Truevision_TGA#Technical_details). But you can force Magick.NET to read it as a specific format with the MagickReadSettings:
MagickReadSettings settings = new MagickReadSettings() { Format = MagickFormat.Tga }; using (MagickImage image = new MagickImage(f, settings)) { }