腾讯体育nba解说嘉宾:问一个vb的问题

在vb中编写了一个打开文件的对话框，如果要使它点击jpg格式的文件时自动用form4进行图片浏览，那么这个步骤的命令应该怎么编写？告诉我源代码！谢谢！（以下是现有的源代码）

Dim setfilename As String
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Private Sub Command1_Click()
RetVal = Shell(setfilename, 1)
End Sub
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Private Sub Command2_Click()
Text1.Text = ""
End Sub
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Private Sub Command3_Click()
msg = MsgBox("确定退出吗？", 4)
If msg = vbYes Then
End
End If
End Sub
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Private Sub Dir1_Change()
File1.Path = Dir1.Path
Text1.Text = File1.Path
End Sub
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Private Sub Drive1_Change()
Dir1.Path = Drive1.Drive
End Sub
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Private Sub Exit_Click()
End
End Sub
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Private Sub File1_Click()
If (Right(File1.Path, 1)) = "\" Then
setfilename = File1.Path & File1.FileName
Text1.Text = Dir1.Path & File1.FileName
Else
setfilename = Dir1.Path & "\" & File1.FileName
Text1.Text = Dir1.Path & "\" & File1.FileName
End If
End Sub
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Private Sub File1_DblClick()
Form4.Show
Unload Form1
RetVal = Shell(setfilename, 1)
End Sub
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Private Sub Form_Load()
Form1.Hide
Form3.Show
End Sub
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Private Sub Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
If Button = 2 Then
PopupMenu File
End If
End Sub
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Private Sub Helpfile_Click()
Unload Form1
Form2.Show
End Sub