Thank you, Henrikhuse, for a most helpful reply to my post.
You have clarified what Parameter 3 and Parameter 5 should do.
In practice, they are not working quite as I expected.
To re-cap my post, I would like to have a file open dialog which allows multiple file selection (which Fileopen.dll does with style=512), and where I can set a mask so only the selected extensions are shown in the file list.
With Fileopen.dll, what I expect to see, when I open the file open dialog (based on the comprehensive documentation in your post) is that all file extensions are masked out except those permitted by Parameter 3 or selected from the option(s) in the extension list box set by parameter 5. In practice, neither Parameter 3 nor 5 seem to influence the list of selectable files. (The Parameter 5 list box seems ineffective and is greyed out, perhaps because it only contains one item. I have not been able work out a format for Parameter 5 to display a list of extensions in the box from which to select.) It may simpply be that, as I don’t know any C, I am not using the correct syntax for these parameters – in which case advice would be much appreciated!
What I find is that I can only mask down the file list to show the preferred extension by passing *.extension as parameter 4.
I also find that I can only set one filename in parameter 4 (Style 512 does not influence Parameter 4) and so I can not mask the file list to show files with either of two defined extensions, which would be a rather useful option.
I can use the Fileopen dll in my application – it is still better than the Widget alternative in that it supports multiple file selection – but it would be much more elegant if I could gain more control of the extension masking in the file list.
Thank you again for your help, and thanks to to all who replied earler to this post.