Merge Request shows "binary file a/file and b/file differ" instead of the diff

I am currently on gitlab-ce omnibus 8.3.3 and have a project where I use combit List&Label as a reporting solution and I store generated reporting projects in the git repository.

List&Label project files are plain text files with the extension .lst In a merge request gitlab shows Binary files a/Reports/Report.lst and b/Reports/Report.lst differ instead of the file content.

I already added

*.lst    text
*.lst    diff=astextplain

to my .gitattributes file but files are still shown as binary.

Howevery, opening the file in the file browser (https://git.company.tld/namespace/project/blob/master/ReportsReport.lst) show the file content rather than a download link like it would do for other "binary" files.

Please respect the projects .gitattributes file for comparing files in merge request.