That said, maybe you should populate the ListBox with both the name and path of the files. If you create a DirectoryInfo for the folder containing the files, call GetFiles and bind the result to the ListBox, you can display only the file names but then get the full path from the selection, e.g.
vb.net Code:
Dim folder As New DirectoryInfo(Path.Combine(Application.StartupPath, myfilespath, "docs"))
Dim files = folder.GetFiles()
With listboxdoc
.DisplayMember = "Name"
.ValueMember = "FullName"
.DataSource = files
End With
vb.net Code:
attachment = New System.Net.Mail.Attachment(CStr(listboxdoc.SelectedValue)) 'file path