To set the image of a button set.Button.image must be used. Adding the block it automatically add also a red block that is similar to a list picker and the list elements are all the files uploaded using upload manager in the Designer view. I also copied and use it in controls that are not buttons and it works both in companion and apk for images, fonts and files (until now I used txt and csv files but I think it works with every other file type) except for sqlite db files, in that case I have to use a text block containing the DB file name preceded by couple of slashes.
I would like to understand if the red block is present only in Buttons or if it is a standard block (I did not find it) and why it does not work with sqlite DB.