RFC: support filetype selection in gui open window and consider b:browsefilter

:h browsefilter says

							*browsefilter*
For MS Windows and GTK, you can modify the filters that are used in the browse
dialog.  By setting the g:browsefilter or b:browsefilter variables, you can
change the filters globally or locally to the buffer.  The variable is set to
a string in the format "{filter label}\t{pattern};{pattern}\n" where {filter
label} is the text that appears in the "Files of Type" comboBox, and {pattern}
is the pattern which filters the filenames.  Several patterns can be given,
separated by ';'.

LibreOffice (tested with v6.1) under macOS supports similar functionality.

Could MacVim support this as well and take b:browsefilter into account?

There are 52 filetype plugins in $VIMRUNTIME which define b:browsefilter.

❯ ag -l browsefilter | wc -l
      52

I noticed that many macOS apps gray out those filetypes they do not support in the open file dialog window but do not provide a pulldown menu to change the filter.

If this is more complicated to implement (non-standard open dialog window), feel free to close this issue since this is not very important for me.

先从 :h browsefilter 和 MacVim GUI 的打开窗口实现开始，然后检查定义 b:browsefilter 的 filetype 插件。完成标准是 macOS 打开对话框支持选择 filetype，并考虑缓冲区本地的 b:browsefilter 值。