I found a Github Repository to Migrate data from Wunderlist to Trello with Python code along with some instructions but I have not tried it myself. Providing the source code works, this might be helpful to those that are comfortable to use source code for the task.
Open the card in Trello and then run the following in your browser inspector console:
// Clicks the "View all attachments" if there are more than four attachments
// Parses for all the attachment ...