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
}
    }
    
    if (typeof config.onDownloadProgress === 'function') {
      request.addEventListener('progress', config.onDownloadProgress);
    }
    
    if (typeof config.onUploadProgress === 'function' && request.upload) {
      request.upload.addEventListener('progress', config.onUploadProgress);
    }
    if (config.cancelToken) {
      
      config.cancelToken.promise.then(function onCanceled(cancel) {
        if (!request) {
          return;
        }
        request.abort();
        reject(cancel);
        
        request = null;
      });
    }
    if (!requestData) {
      requestData = null;
    }
    
    request.send(requestData);
  });
};