Hi @gpdmop @lonetour, don’t know if you’re still having trouble with your list of files. I just had the same problem with my first app but I’ve found a solution (little bit creepy but efficient), have a look :
Make a Workflow with one step for each file, step by step, one by one. (I’ve limited upload to 20 files/image, fairly enough I think).
Each step adds one of the listed files into the image list, here is the first step :
“[Your image list] add [index]'s Uploaded files:item #1’s URL”
Add a condition :
“Only when [index]'s uploaded files:count ≥ 1”
Following steps :
“[Your image list] add [index]'s Uploaded files:item #[current image step]'s URL”
- condition “Only when [index]'s uploaded files:count ≥ [current image step]”
Then duplicate this condition as many time as needed. Then just modify the [current image step] number. Your list of files is now a list of images, without any blank one 
PS : Sorry for my poor english 