I think I found out the allowed file formats for Amazon Prime Photos Plan:

*.jpg; *.JPG; *.CR2; *.ORF; *.tff; *.tif; *.jpeg; *.JPEG; *.dng; *.RW2; *.gif; *.GIF; *.png; *.PNG; *.bmp; *.BMP

Not allowed:

My .RAW files from my Panasonic camera are not recognized, and therefor not allowed!

Prime Photos Plan, Prime Photos, allowed, not allowed