Use a more suitable content type for file uploads

application/x-www-form-urlencoded;charset=UTF-8 is the default, which shinyapps.io
cares about for some reason and tries to parse the data as such. By setting the
content type to the more accurate application/octet-stream, no middleware should
be tempted to futz with the contents.

srcjs/input_binding_fileinput.js

@@ -107,6 +107,7 @@ $.extend(FileUploader.prototype, FileProcessor.prototype);
         return xhrVal;
       },
       data: file,
+      contentType: 'application/octet-stream',
       processData: false,
       success: function() {
         self.progressBytes += file.size;