nsITransferable
void settransferdata( in string aflavor, in nsisupports adata, in unsigned long adatalen ); parameters an instance of a class based upon nsisupportsprimitives an nsifile an nsiflavordataprovider aflavor the flavor of data that is being set.
... adatalen the length of the data, or 0 if adata is an nsiflavordataprovider.
... see also nsiflavordataprovider nsisupportsprimitives nsiformatconverter ...

Recommended Drag Types - Web APIs
ntevent.datatransfer.setdata("application/x-moz-file-promise-dest-filename", leafname); currentevent.datatransfer.mozsetdataat('application/x-moz-file-promise', new dataprovider(success,error), 0, components.interfaces.nsisupports); function dataprovider(){} dataprovider.prototype = { queryinterface : function(iid) { if (iid.equals(components.interfaces.nsiflavordataprovider) || iid.equals(components.interfaces.nsisupports)) return this; throw components.results.ns_nointerface; }, getflavordata : function(atransferable, aflavor, adata, adatalen) { if (aflavor == 'application/x-moz-file-promise') { var urlprimitive = {}; var datasize = {}; atransferable.gettransferdata('application/x-moz-file-promise-url', url...