As shown in the sample from the docs, the AddPattern
method is meant for adding a single pattern at a time. In contrast to the WinForms implementation and similar implementations, it does not set all patterns at a time, instead it adds one additional pattern to whatever has been added before.
Therefore, try splitting up your command:
fc.SelectMultiple = true;
FileFilter filter = new FileFilter();
filter.Name = "Image files";
filter.AddPattern("*.jpg");
filter.AddPattern("*.jpeg");
filter.AddPattern("*.png");
filter.AddPattern("*.tif");
filter.AddPattern("*.bmp");
filter.AddPattern("*.gif");
filter.AddPattern("*.tiff");
fc.Filter = filter;