Board Thread:Support Requests - Designing Your Wiki/@comment-3361105-20130202021527/@comment-3335785-20130202023032

From what I can tell the alt attribute on a file page is set to whatever the file is named. For example if the file was named Example12 3.png, it's alt would be set to File:Example12 3.png. I imagine it's a mediawiki default that we can't really change on a case by case basis without some scripting. Whether the script would really be worth it, who knows.

You can always edit the File page itself to set a description of what the file is for those unaware.