diff --git a/activestorage/app/assets/javascripts/activestorage.js b/activestorage/app/assets/javascripts/activestorage.js
index e2bcb520b9..6061fb9bb6 100644
--- a/activestorage/app/assets/javascripts/activestorage.js
+++ b/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
       };
diff --git a/activestorage/app/javascript/activestorage/blob_record.js b/activestorage/app/javascript/activestorage/blob_record.js
index 7fbe315f76..997c123870 100644
--- a/activestorage/app/javascript/activestorage/blob_record.js
+++ b/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
     }