Drag/drop file upload (storage file upload form type)

Requirements

• When adding a file (content type based on document type file) you now see a standard upload field. This should be a drag/drop upload using http://www.dropzonejs.com/ (if you know a better one please discuss with the PO)
• You'll need to create a custom formtype for this.
• (don't forget to use bower to install it)
• Make sure validation works correct, because it doesn't work at this moment: when the file field is set to required in the content type configuration a file upload is required, unless there is an existing file (existing files are currently not detected when set to required)

Technical implementation

• A pull request is already available, but we don't know if the code is complete and OK. Because the code can be used, maybe not

Test scenarios

• Create a new File or Image content item and drap / drop a file in the latest IE, Chrome, Safari and VuurFox
• Set File to "not required" --> try to add a content item without a flie
• Set File to "required" --> try to add a content item without a flie, try it with a file and try to edit an existing item, without uploading a new file
• Open existing file --> is image still displayed / file download link still available