[Dropzone] Inform the user of the number of files he uploads

[rrr63]

Q	A
Bug fix?	no
New feature?	yes
Docs?	no
Issues	#2567
License	MIT

Hi, I was implementing a multiple file dropzone in one of my projects, and i was facing the same problem mentioned in #2567.
I was thinking for a solution, should we make a preview of all files if they're images ?
Sounds an interesting idea first, but if users upload a lot of images, it could be very ugly to manage the preview. Also, i was worried about avoiding regression for projects that are already in production and to break some custom dropzone styles.

The solution i used for my project is to show the number of extra files user uploads. I was inspired by the original file type field, it just shows us the number of files, according to the stylized dropzone of symfony ux, the first element (if it is an image) display a preview, and we write to user the number of extra files he uploads.

Here a screenshot of three fields :
-dropzone with single file
-dropzone with 3 files
-standard file type field with 3 files

[github-actions]

📊 Packages dist files size difference

Thanks for the PR! Here is the difference in size of the packages dist files between the base branch and the PR.
Please review the changes and make sure they are expected.

File	Before (Size / Gzip)	After (Size / Gzip)
Dropzone
controller.js	3.12 kB / 816 B	3.39 kB+8% 📈 / 913 B+12% 📈