understanding of to upload more than one image in a gallery

Hello

I have shown a user how to manage articles in the background.
He has not understood one thing about the image/gallery field when more than one image is uploaded.

I think it's mainly a translation problem :
to upload an image we have the button at the right side 'add a file' in french 'ajouter une image'. No problem for the first image, but for the second one it has been difficult to explain him to click at the bottom of the current image to add a new file and then at the right side of the second image to upload the second one.
He has directly clicked at the right of the first image 'add a file' and replaced the first image.

Of course the best thing would have been that the bottom button 'add' open directly the popup form allowing adding image.
But, if this is not possible, I propose another sentence on the right button : replacing 'add a file' by 'Choose a file' would be easier to understand.
In french the sentence 'ajouter un fichier' may be replaced by 'choisir un fichier'.

I have searched in transifex where is the sentence but I didn't found it.

best regards

Michel

I see, then i will use new language strings, to avoid confusion