If the name of an uploaded object does not begin with letter, it will be prefixed with an 'x'. E.g. an object that would normally be called _namespace_tabname.csv will now be called x_namespace_tabname.csv when uploaded to S3, Azure Storage or Google Cloud Storage.
This change is to make sure that all uploaded object are compatible with Azure Data Factory, Hadoop and Spark, which require object names to begin with a letter or give special meaning to objects whose name starts with certain non-alphabetic characters. E.g. in Spark all objects that begin with an _ or . are hidden.