Dear Zijkant,
There isn't a known function to handle this,
uploaded files: picProj1_1.jpg / picProj1_2.jpg / picProj1_3.jpg.
These are file name of uploaded files, picproj1 is the file prefix, you can write code to get the "_n" . For example, you can read target folder, If you only find one existing file in target folder, you can call next file as picProj1-2.jpg ;
If you find three existing files in target folder, you can call next file as picProj1-4.jpg.
Thank you for asking