title: Pandas Conditional new column based on period found in other dataframe column
body: <p>I have a dataframe with file extensions.  Some have periods in them I am trying to create a new column flagging which ones contain a period or not conditionally.  If I wanted to just get the rows that contain a period I would just use: <code>send_rec_file_url[send_rec_file_url['file_name'].str.contains('\.')]</code>.  </p>
  
  <p>How do I create a new column like below?</p>
  
  <pre><code>df
      file_name
  0   png 
  1   jpg
  2   jpg
  3   pdf
  4   pdf
  5   xlsx
  6   docx.pdf
  7   txt.scf
  8   pdf
  9   TXT.vbs
  10  read_this.pdf 
  </code></pre>
  
  <p>Desired output:</p>
  
  <pre><code>df
      file_name      has_period
  0   png            no
  1   jpg            no
  2   jpg            no
  3   pdf            no
  4   pdf            no
  5   xlsx           no
  6   docx.pdf       yes
  7   txt.scf        yes
  8   pdf            no
  9   TXT.vbs        yes
  10  read_this.pdf  yes
  </code></pre>