Defaults content_type to application/octet-stream

Defaults content_type to application/octet-stream in blob_record.js

[direct_upload_xls_in_chrome]

activestorage/app/assets/javascripts/activestorage.js

@@ -550,7 +550,7 @@
       this.file = file;
       this.attributes = {
         filename: file.name,
-        content_type: file.type,
+        content_type: file.type || "application/octet-stream",
         byte_size: file.size,
         checksum: checksum
       };

activestorage/app/javascript/activestorage/blob_record.js

@@ -6,7 +6,7 @@ export class BlobRecord {
 
     this.attributes = {
       filename: file.name,
-      content_type: file.type,
+      content_type: file.type || "application/octet-stream",
       byte_size: file.size,
       checksum: checksum
     }