it should be simple but as usual something goes wrong :-(
B4X:
Dim fc As FileChooser
Dim fileSelected As String 'File selected. If selected full path is returned else ""
fc.Initialize 'Init! Do not forget
fc.InitialDirectory = File.DirApp 'Set starting folder
fc.Title = "Select a File"
fileSelected = fc.ShowOpen(MainForm) 'Show the dialog and select
If fileSelected.Length > 0 Then
SetFieldValue(fileSelected)
WebView1.LoadUrl("file:///" & fileSelected.Replace(" ","%20").Replace("\","/"))
End If