Hello Will,
The issue is because of the security setting for Internet Explorer
Here is way to change the full file path to only the file name:
1. Open Internet Explore and click Tools --> Internet Options --> Security --> Custom level button
2. Confirm that disable "Include local directory path when uploading files to a server." is checked
3. Click OK and try to attach new file. The issue should be fixed.
Hope this can help.
Best Regards,