I am currently working on the reviews module where I added a multiple file upload on the review form. I was able to add the field in storefront and create a custom table for the multiple review image saving and was able to make it work. The remaining issue is how do I add an upload field (preferably with preview) for both add and edit review form in admin.
You can create file-uploader element in your custom admin form and you can add and edit data from there
You can follow this link
Hope this will work for you