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 (xsrfValue) {
        requestHeaders[config.xsrfHeaderName] = xsrfValue;
      }
    }
    
    if ('setRequestHeader' in request) {
      utils.forEach(requestHeaders, function setRequestHeader(val, key) {
        if (typeof requestData === 'undefined' && key.toLowerCase() === 'content-type') {
          
          delete requestHeaders[key];
        } else {
          
          request.setRequestHeader(key, val);
        }
      });
    }
    
    if (!utils.isUndefined(config.withCredentials)) {
      request.withCredentials = !!config.withCredentials;
    }
    
    if (config.responseType) {
      try {
        request.responseType = config.responseType;
      } catch (e) {
        
        
        if (config.responseType !== 'json') {
          throw e;
        }