Why can't I open file:// links?
Trello attachments work one of two ways: by either uploading a copy of the file to our servers (for attachments added from your computer or mobile device) or by linking to an attachment on another service’s servers (for URL attachments, Dropbox attachments, etc.).
Currently, Trello does not allow attaching local files without them being uploaded to Trello’s servers. This means that shared drive attachments, attachments stored on an FTP server, and other local file storage options can’t be used with Trello without uploading those files to Trello’s servers. As a result, additional changes made to the file in your local shared drive won’t be reflected in Trello.
If you need a file stored in a local file sharing service to reflect changes that you make to it, there are a few options.
Use a cloud storage system
Trello does support file attachments from Google Drive, OneDrive, and Dropbox. If you have access to those services, you can upload your file there. Since those Trello attachments work as links, the attached file will always link to the most recent version in the cloud storage solution you’re using.
Paste in a Link
If you need to reference a specific shared drive file, you can paste a link to that file (such as file://F/image.png) in a card comment or description.
When that link is clicked on, Trello will prompt you to copy the link and paste it in to your own browser. (For security reasons, Trello is not able to directly open file protocols other than http://, https://, or mailto:.) Your computer will then try to open the file.
For ftp:// links specifically, your browser should try to open them directly if you click on them in the card description or comments.