LibCore: Set mime type for .sheets files to `application/x-sheets+json`

This is our own format, used by Spreadsheet.

Userland/Libraries/LibCore/MimeData.cpp

@@ -89,6 +89,8 @@ String guess_mime_type_based_on_filename(StringView path)
         return "text/html";
     if (path.ends_with(".csv", CaseSensitivity::CaseInsensitive))
         return "text/csv";
+    if (path.ends_with(".sheets", CaseSensitivity::CaseInsensitive))
+        return "application/x-sheets+json";
     // FIXME: Share this, TextEditor and HackStudio language detection somehow.
     auto basename = LexicalPath::basename(path);
     if (path.ends_with(".cpp", CaseSensitivity::CaseInsensitive)