Option to select one more files to download

I'm not sure where you're reading that but the multi file upload absolutely let you select multiple files all at once. That's the whole point of a multi file upload. So I suggest you re-read the instructions or post where it says that because that's not true.  

Having said that the other thing I meant to say is that Juiced Technologies also has a file manager multi file upload

https://www.juicedtech.com/file-attachment-multifile-upload

and they've had that for like 20 years or a long time at least.  Yes there is a cost to it but it does have some advantages over the native Quickbase plug-in in that you can tag pictures as they're uploaded. A typical use case would be in a real estate environment for a realtor or a home inspection and you want a convenient drop down liost of room types like "Basement" or a "Primary Bedroom" etc and as the files are uploaded they would be tagged in a multiple choice field.  So the idea would be to grab all the pictures of the primary bedroom off your camera roll on your phone and tag them primary bedroom and upload those and then move onto a batch of pictures for the basement.    Obviously these tools were written by a different developers so the actual user interface will be different. There is a free 30 day trial with Juiced you so if you want you can try them both and decide which one is better for your needs and your budget.

I know with the Juiced tool once the multi file upload is complete and you close the box it automatically refreshes as the parent records so the user sees the files uploaded. I forget if the native plug-in has that convenient behavior.   I seem to recall doing an application that was going be used by very in-frequent EOTI users and we switched over to the Juiced add on because it was not obvious to the users using the native QuickBase plug-in method that the files actually were uploaded.