OK thanks you all for the kind replies.
I've got one more question: I tried to manually move a file to the download folder with the stock file manager, and then I went to protonmail app trying to add the file as an attachment. I was surprised when the app could see the file, although it wasn't created by the app itself! Could it be that the selection of the file is made through the stock file manager (that I assume has access to almost all files)?