Fix #3983: Uploads: NoMethodError - undefined method `>=' for nil:NilClass

* Rename Upload#download_for_upload to #get_file_for_upload.

* Fix #get_file_for_upload to raise error if no file or source url was given.

* Fix javascript upload validation to disallow submitting form if file is
  not present and the source is not an url.

app/logical/upload_service/utils.rb

@@ -206,7 +206,10 @@ class UploadService
       tags.join(" ")
     end
 
-    def download_for_upload(upload)
+    def get_file_for_upload(upload, file: nil)
+      return file if file.present?
+      raise RuntimeError, "No file or source URL provided" if upload.source_url.blank?
+
       attempts = 0
 
       begin