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?

Reproduce the drag-and-drop flow on the Iodide main page with Furniture_only.xls, then trace how the dropped file creates the notebook fetch directive and chooses text versus blob. Done means binary files default to blob while the shown fetch behavior remains correct for text files.

Written by the indexing model from the issue text.