Here is the error case: <https://codepen.io/Gerhut/pen/OErpQm?editors=0012>

Browsers will set Content-Type header automatically from `Blob#type` or `File#type` (if exists) when sending a Blob/File through xhr, which is overwritten by `axios.defaults.headers.post`.

Shall we provide a specific behavior about this case, either remove the `Content-Type` and let it be set by the browser (like FormData) or set it as `Blob#type` or `File#type` manually to increase compatibility? (Although I don't know if there is any browser don't set `Blob#type` or `File#type` in xhr.)

lib/adapters/xhr.js
request = null;
    };
    
    request.onerror = function handleError() {
      
      
      reject(createError('Network Error', config, null, request));
      
      request = null;
    };
    
    request.ontimeout = function handleTimeout() {
      var timeoutErrorMessage = 'timeout of ' + config.timeout + 'ms exceeded';
      if (config.timeoutErrorMessage) {
        timeoutErrorMessage = config.timeoutErrorMessage;
      }
      reject(createError(timeoutErrorMessage, config, 'ECONNABORTED',
        request));
      
      request = null;
    };
    
    
    
    if (utils.isStandardBrowserEnv()) {
      
      var xsrfValue = (config.withCredentials || isURLSameOrigin(fullPath)) && config.xsrfCookieName ?
        cookies.read(config.xsrfCookieName) :
        undefined;