Fix export upload

doc/api/project_import_export.md

@@ -42,6 +42,9 @@ curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" https://gitlab
 }
 ```
 
+NOTE: **Note:**
+The upload request will be sent with `Content-Type: application/gzip` header. Ensure that your pre-signed URL includes this as part of the signature.
+
 ## Export status
 
 Get the status of export.

lib/gitlab/import_export/after_export_strategies/web_upload_strategy.rb

@@ -52,7 +52,10 @@ module Gitlab
         end
 
         def headers
-          { 'Content-Length' => export_size.to_s }
+          {
+            'Content-Type' => 'application/gzip',
+            'Content-Length' => export_size.to_s
+          }
         end
 
         def export_size