16 bit landscape brush alpha

I’m having a problem with the brush alpha for the landscape sculpting tool.
I’m only able to import a JPEG file in RGB mode and no other image type, I want to know if it’s possible to import a PNG for example, because I want to use the full 16 bit color range from my original alpha and not be limited to 8 bit from the JPEG file

bumb

is it maybe just supposed to work only with 8bit jpeg?