I've never used Geocities but the choose file option should be the same as in gmail,forums that allow uploading, or any website that allows uploading.

When you click onto it, for me on my cb it opens my cb Download folder, where you can then click onto a file in that folder and choose to Open it, which will then upload that file in your case to Geocities.

When it opens the Download folder, you don't have to choose a file from that folder, you can click onto one of the folders listed in the left menu and that folder will open so that you can choose a file from the folder you chose.

Choose File has always been a way to upload files from your cb/pc/ipad or whatever electronic that has storage on it.