The documentation I have for this call is the first screenshot I share 
The url looks like this :
POST : api/declarations/{idDeclaration}/photos/
I can’t give much more information than that
, or don’t know what information you would need to further help (let me know !)
The understanding I have is that form-data is meant to give a solution to send files in POST requests, which JSON can’t do.
How do you usually send files via POST requests ? Do you do it directly in the JSON body ?
edit : the API is a private one I can’t really share it, wouldn’t add much more information