Add Drag & Drop support for binary and attachment
On tryton, we can drop file on attachment button but this is missing on sao. We could convert the button into a drop zone[1].
On sao, the input file support to drop file on it (on most browsers) but this is missing on tryton. We could connect the select button to drag-data-received.
[1] https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API/File_drag_and_drop