Are you using Paste Email (Original) or Paste Email Plus
? I just tried it with both and it works as expected.
Basically, you need to focus on an input field (by clicking on it), right-click to open the context menu, then select Paste Email > Paste from file
. An file open dialog window should appear where you can navigate to any text file. When you click the file name and click the 'Open' button, the contents should paste into the input field.