Opening other desktop apps using a callback URL attached to a card doesn't open the target app

Issue Summary

Users can use the attachment feature to attach files, images, and links referencing places outside of Trello. However, opening a callback URL that's attached to a card through the Desktop app to open a different app on the same device no longer works as expected. 

Steps to Reproduce

1. Open a card
2. Click on attachments 
3. Paste a callback URL for the app you want to open (for example, spotify://open-spotify?playlist=PLAYLIST_ID)
4. The attachment won't allow you to attach this type of link unless you add a forward slash ("/") at the beginning of the callback URL (for example, /spotify://open-spotify?playlist=PLAYLIST_ID)
5. Then, edit the link attached to the card back to its original form (spotify://open-spotify?playlist=PLAYLIST_ID).

Expected Results

Clicking on the attached callback URL should open the specified Desktop app upon clicking on it.

Actual Results

Clicking on the attached callback URL doesn't open the specified Desktop app when using Trello Desktop app. 

Workaround

Users can use their browser to open these links, which will reference the Desktop app they want to open. They also need to remember to add the forward-slash ("/") at the beginning of the callback URL to attach the link, and then edit the link to remove it, ensuring the callback URL will work as expected.