Diff later not using configured Diff-Viewer
What steps will reproduce the problem?
What is the expected output? What do you see instead?
It should diff the two files with the alternative Diff Viewer but will diff them with TortoiseGitMerge instead.
What version of TortoiseGit and Git are you using? On what operating system?
188.8.131.52, Git should not be of any importance.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information