Fixed isFormData predicate;

[DigitalBrainJS]

• Fixed isFormData util; (possibly related to Cannot upload file within FormData #4412, Can't post FormData since Axios 0.25.0 #4406). It can detect (at least) native FormData instance, form-data, formdata-node
• Added support for automatic object serialization to FormData if Content-Type is multipart/form-data; (toFormData helper function #3757)
• Added support for FormData to be overloaded using config.env.FormData option (for node.js environment);
• Added support for FormData in node.js environment through form-data package (the package is not a dependency);

Browser example:

const axios= require('axios');

axios.post('https://postman-echo.com/post', {x: 1}, {
  headers: {
    'Content-Type': 'multipart/form-data'
  }
}).then(({data})=> console.log(data));

Node.js example

const axios= require('axios');
var FormData = require('form-data');

// OR
// axios.defaults.env.FormData= FormData;

axios.post('https://postman-echo.com/post', {x: 1}, {
  headers: {
    'Content-Type': 'multipart/form-data'
  },

  env: {  // OR
    FormData
  }
}).then(({data})=> console.log(data));

Response:

{
  args: {},
  data: {},
  files: {},
  form: { x: '1' },
  headers: {
    'x-forwarded-proto': 'https',
    'x-forwarded-port': '443',
    host: 'postman-echo.com',
    'x-amzn-trace-id': 'Root=1-61eb41dc-5eb39a192efeea3a65d9e624',
    'content-length': '157',
    accept: 'application/json, text/plain, */*',
    'content-type': 'multipart/form-data; boundary=--------------------------086549356411608332106674',
    'user-agent': 'axios/0.25.0'
  },
  json: null,
  url: 'https://postman-echo.com/post'
}

[carpben]

I must say I'm a bit concerned about this PR.
Especially since it was added with out any tests. @jasonsaayman I suggest waiting with releasing the next version until proper tests are added.