diff --git a/pillar/application/modules/file_storage.py b/pillar/application/modules/file_storage.py
index 877572d4..74e36060 100644
--- a/pillar/application/modules/file_storage.py
+++ b/pillar/application/modules/file_storage.py
@@ -4,6 +4,7 @@ import mimetypes
 import os
 import tempfile
 import uuid
+import io
 from hashlib import md5
 
 import bson.tz_util
@@ -546,6 +547,14 @@ def stream_to_gcs(project_id):
         local_file = None
         stream_for_gcs = uploaded_file.stream
 
+    # Figure out the file size, as we need to pass this in explicitly to GCloud.
+    # Otherwise it always uses os.fstat(file_obj.fileno()).st_size, which isn't
+    # supported by a BytesIO object (even though it does have a fileno attribute).
+    if isinstance(stream_for_gcs, io.BytesIO):
+        file_size = len(stream_for_gcs.getvalue())
+    else:
+        file_size = os.fstat(stream_for_gcs.fileno()).st_size
+
     # Upload the file to GCS.
     from gcloud.streaming import transfer
     # Files larger than this many bytes will be streamed directly from disk, smaller
@@ -554,7 +563,7 @@ def stream_to_gcs(project_id):
     try:
         gcs = GoogleCloudStorageBucket(project_id)
         blob = gcs.bucket.blob('_/' + internal_fname, chunk_size=256 * 1024 * 2)
-        blob.upload_from_file(stream_for_gcs,
+        blob.upload_from_file(stream_for_gcs, size=file_size,
                               content_type=uploaded_file.mimetype)
     except Exception:
         log.exception('Error uploading file to Google Cloud Storage (GCS),'