All right, let's go with a feedback. First of all, I've got technical issues handling with this tool as follows (after attemping to add another texture):
A generic error occured in GDI+.************** Exception Text **************
System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.
at System.Drawing.Image.Save(String filename, ImageCodecInfo encoder, EncoderParameters encoderParams)
at System.Drawing.Image.Save(String filename, ImageFormat format)
at Multifshview_test.Multifshfrm.RefreshBitmapList()
at Multifshview_test.Multifshfrm.addbtn_Click(Object sender, EventArgs e)
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
************** Loaded Assemblies **************
mscorlib
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll
----------------------------------------
Multifshview
Assembly Version: 1.3.0.0
Win32 Version: 1.3.0.0
CodeBase: file:///F:/sc%20work/__Tools/MultiFsh/Multifshview.exe
----------------------------------------
System.Windows.Forms
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
System.Drawing
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.1433 (REDBITS.050727-1400)
CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
FSHLib
Assembly Version: 0.3.1.32075
Win32 Version: 0.3.1.32075
CodeBase: file:///F:/sc%20work/__Tools/MultiFsh/FSHLib.DLL
----------------------------------------
Clicking "continue" allows to work, but I'm not sure if everything is ok, probably not - choosing bitmap #2 or more gives another error box:
Index was outside the bounds of the array.
Beside this, some of my advice (and wishes, too), how this multifsh tool should work. First of all, making base/overlay textures should handle 5 FSH files at once. I realise you probably know it, but for sure I'll explain. Every zoom bitmap is stored in different FSH file as TGI follows:
0xXXXXXXX
0-4 (0 is the smallest, 4 is the biggest),
0xXXXXXXX
5-9 (5 is the smallest, 9 is the biggest),
0xXXXXXXX
A-E (A is the smallest, E is the biggest).
Now, generate mipmaps should add proper downscaled bitmap in proper FSH file, on the proper layer (slot 0-3 for multi-textures - I haven't tried if more textures work - I shall make some tests). That way it would be better to handle .dat instead of .fsh file. The tool should allow only to choose the texture using TGI with digit 0, 5, A.
I know it does change your tool much, null45. I don't expect you'll agree to make this, although it would be very appreciated -I'm going to use multitextures pretty intensively

.
Anyway, thanks for the time you've spent!