这里提供自己解析的一个工具：
http://www.delegate.org/delegate/
http://www.delegate.org/delegate/download/
http://www.delegate.org/delegate/sample/data-url.html

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Option Explicit

'需要引用Microsoft XML, v3.0
Private Function Encode(iArray() As Byte) As String
Dim iXml As New MSXML2.DOMDocument30
With iXml.createElement("Encoder")
.dataType = "bin.base64"
.nodeTypedValue = iArray()
Encode = .Text
End With
End Function


Private Function Decode(ByVal iStrbase64 As String) As Byte()
Dim strXML As String
strXML = "& Chr(34) & "urn:schemas-microsoft-com:datatypes" & Chr(34) & " dt:dt=" & Chr(34) & "bin.base64" & Chr(34) & ">" & iStrbase64 & ""
With New MSXML2.DOMDocument30
.loadXML strXML
Decode = .selectSingleNode("DECODER").nodeTypedValue
End With
End Function

Public Function EncodeBase64(ByVal vsFullPathname As String) As String
'For Encoding BASE64
Dim b As Integer
Dim Base64Tab As Variant
Dim bin(3) As Byte
Dim s As String
Dim l As Long
Dim i As Long
Dim FileIn As Long
Dim sResult As String
Dim n As Long

'Base64Tab=>tabla de tabulaci髇
Base64Tab = Array("A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z", "a
", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "+", "/")

Erase bin
l = 0: i = 0: FileIn = 0: b = 0:
s = ""

'Gets the next free filenumber
FileIn = FreeFile

'Open Base64 Input File
Open vsFullPathname For Binary As FileIn

sResult = s & vbCrLf
s = ""

l = LOF(FileIn) - (LOF(FileIn) Mod 3)

For i = 1 To l Step 3

'Read three bytes
Get FileIn, , bin(0)
Get FileIn, , bin(1)
Get FileIn, , bin(2)

'Always wait until there're more then 64 characters
If Len(s) > 64 Then

s = s & vbCrLf
sResult = sResult & s
s = ""

End If

'Calc Base64-encoded char
b = (bin(n) \ 4) And &H3F 'right shift 2 bits (&H3F=111111b)
s = s & Base64Tab(b) 'the character s holds the encoded chars

b = ((bin(n) And &H3) * 16) Or ((bin(1) \ 16) And &HF)
s = s & Base64Tab(b)

b = ((bin(n + 1) And &HF) * 4) Or ((bin(2) \ 64) And &H3)
s = s & Base64Tab(b)

b = bin(n + 2) And &H3F
s = s & Base64Tab(b)

Next i

'Now, you need to check if there is something left
If Not (LOF(FileIn) Mod 3 = 0) Then

'Reads the number of bytes left
For i = 1 To (LOF(FileIn) Mod 3)
Get FileIn, , bin(i - 1)
Next i

'If there are only 2 chars left
If (LOF(FileIn) Mod 3) = 2 Then
b = (bin(0) \ 4) And &H3F 'right shift 2 bits (&H3F=111111b)
s = s & Base64Tab(b)


b = ((bin(0) And &H3) * 16) Or ((bin(1) \ 16) And &HF)
s = s & Base64Tab(b)

b = ((bin(1) And &HF) * 4) Or ((bin(2) \ 64) And &H3)
s = s & Base64Tab(b)

s = s & "="

Else 'If there is only one char left
b = (bin(0) \ 4) And &H3F 'right shift 2 bits (&H3F=111111b)
s = s & Base64Tab(b)

b = ((bin(0) And &H3) * 16) Or ((bin(1) \ 16) And &HF)
s = s & Base64Tab(b)

s = s & "=="
End If
End If

'Send the characters left
If s <> "" Then
s = s & vbCrLf
sResult = sResult & s
End If

'Send the last part of the MIME Body
s = ""

Close FileIn
EncodeBase64 = sResult

End Function