In every email client, there should be an Outbox and Sent folders. Typically, the file that is about to be sent is located in the Outbox folder. You can find it in the folder tree on the left. Locate it, right click it and choose delete, then use Synchronize to remove it from the queue as well.
Note that some email services don't allow files bigger than 25 MB. You might want to use Google Drive or Dropbox to send the file.