Binary dragged & dropped files should not be fetched as text by default

Steps to reproduce:

• drag & drop a binary file into iodide main page (e.g. an excel file called Furniture_only.xls)

Expected:

%% fetch
blob: file0=Furniture_only.xls

Got:

%% fetch
text: file0=Furniture_only.xls

It is still a pain to do something with the excel file (#2447) but the fact that we're fetching the file as text is yet another roadblock. Perhaps we could sniff the type of the uploaded file and then set this property of the created notebook accordingly? @hamilton / @openjck any thoughts on this?