title: dynamic file format in dynamic datasets
body: <p>I configured my ADLS2 Dataset as a Parquet format and partition my data by YY-MM-DD-HH like this.</p>
  <p>@{item().Table_Name}/@{formatDateTime(utcnow(),'yyyy')}/@{formatDateTime(utcnow(),'MM')}/@{formatDateTime(utcnow(),'dd')}/@{item().Table_Name}@{formatDateTime(utcnow(),'HH')}</p>
  <p>it's working fine and creates the filename in this folder like in each copy activity in for each table: table/2021/04/04/table12</p>
  <p>I want to add '_' in my final file,i tried use concat function but it's not resolving correct format.</p>
  <p>wanted structure.</p>
  <p>table/2021/04/04/table_12.parquet</p>