Option Explicit

Private Const HKEY_CLASSES_ROOT = &H80000000
Private Const KEY_QUERY_VALUE = &H1
Private Const REG_SZ = 1
Private Const ERROR_SUCCESS = 0&
Private Declare Function RegCloseKey Lib "advapi32.dll" (ByVal hKey As Long) As Long
Private Declare Function RegOpenKeyEx Lib "advapi32.dll" Alias "RegOpenKeyExA" _
  (ByVal hKey As Long, ByVal lpSubKey As String, ByVal ulOptions As Long, _
  ByVal samDesired As Long, phkResult As Long) As Long
Private Declare Function RegQueryValueEx Lib "advapi32.dll" Alias "RegQueryValueExA" _
  (ByVal hKey As Long, ByVal lpValueName As String, ByVal lpReserved As Long, _
  lpType As Long, lpData As Any, lpcbData As Long) As Long

Private Sub cmdBrowse_Click()
  dlg.ShowOpen
  txtFile = dlg.FileName
  If InStr(txtFile, ".") > 0 Then
    txtContentType = GetContentType(Mid(txtFile, InStrRev(txtFile, ".")))
  Else
    txtContentType = "application/octet-stream"
  End If
End Sub

Private Function GetContentType(ByVal s As String) As String
  Dim hKey As Long
  Dim lpSubKey As String
  Dim lpValueName As String
  Dim lpType As Long
  Dim lpData As String
  Dim lpcbData As Long
  
  GetContentType = "application/octet-stream"
  lpSubKey = "Content Type"
  If RegOpenKeyEx(HKEY_CLASSES_ROOT, s, 0, KEY_QUERY_VALUE, hKey) = ERROR_SUCCESS Then
    RegQueryValueEx hKey, lpSubKey, 0, lpType, Chr(0), lpcbData
    If lpType = REG_SZ Then
      lpData = Space(lpcbData)
      If RegQueryValueEx(hKey, lpSubKey, 0, lpType, ByVal lpData, lpcbData) = ERROR_SUCCESS Then
        GetContentType = Left(lpData, lpcbData)
      End If
    End If
    RegCloseKey hKey
  End If
End Function