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condition: {
  // ÏÞÖÆÉÏ´«Îļþ±ØÐëСÓÚ 5MB
  'numeric_less_than_equal': {
    'cos:content-length': 5 * 1024 * 1024
  },
 }

  var policy = JSON.stringify({
    ...
    conditions: [
      ['content-length-range', 1, 5 * 1024 * 1024], // ¿ÉÏÞÖÆÉÏ´«Îļþ´óС·¶Î§±ÈÈç1 - 5MB
    ],
  });

condition: {
  // ÏÞÖÆÉÏ´«Îļþ content-type ±ØÐëΪͼƬÀàÐÍ
  'string_like': {
    'cos:content-type': 'image/*'
  }
 }

  var policy = JSON.stringify({
    ...
    conditions: [
      // ÏÞÖÆÉÏ´«Îļþ content-type ±ØÐëΪͼƬÀàÐÍ
      ['starts-with', '$Content-Type', 'image/*'],
    ],
  });

/** ·þÎñ¶ËÉú³ÉÉÏ´«Â·¾¶Ê¾Àý nodejs **/ 
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// »ñÈ¡Ç°¶Ë´«½øÀ´µÄÎļþºó׺ ext
const ext = req.query.ext;
const cosKey = generateCosKey(ext);
?
function generateCosKey(ext) {
  const date = new Date();
  const m = date.getMonth() + 1;
  const ymd = `${date.getFullYear()}${m < 10 ? `0${m}` : m}${date.getDate()}`;
  const r = ('000000' + Math.random() * 1000000).slice(-6);
  const cosKey = `file/${ymd}/${ymd}_${r}${ext ? `.${ext}` : ''}`;
  return cosKey;
};