multipart/form-data | 含有文件提交的表单请求

通常会看到表单提交带有文件的时候，一般content-type长这样：
multipart/form-data; boundary=----WebKitFormBoundaryV1xLBsu5DDg4628C
那么boundary对应值的后半部分是怎么来的呢，搜了一下chromium的代码，找到了答案。
下面是用python对应写的，主要是python的requests模块boundary的样子不一样，强迫症~

from random import randint
kAlphaNumericEncodingMap = [0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4A, 0x4B, 0x4C, 0x4D, 0x4E, 0x4F, 0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5A, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6A, 0x6B, 0x6C, 0x6D, 0x6E, 0x6F, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7A, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x41, 0x42]
random_chars = "".join([chr(kAlphaNumericEncodingMap[randint(0,63)]) for _ in range(16)])
boundary = "----WebKitFormBoundary" + random_chars
print(boundary)