cypress-file-upload: [Bug] Support Firefox (Cypress 4.0.1)

Current behavior:

I have a test to upload a file (image/png) and it’s been working fine on Chrome with Cypress 3.x and the [new release 4.0](https://cypress.io/blog/2020/02/06/introducing-firefox-and-edge-support-in-cypress-4-0/9. But when I run it on Firefox (with Cypress 4.0.1) it doesn’t work anymore.

• Firefox Developer Edition 73

Desired behavior:

• Chrome 79 (the test is passing on Cypress 3.x and Cypress 4.0.1

Steps to reproduce: (app code and test code)

The HTML is like this:

  <label>
    <img src="image.png" alt="">
    <span>Change avatar</span>
    <input type="file" name="picture" accept="image/*" placeholder="http://" data-test="avatar" class="sr-only">
  </label>
  <div>
      <i class="icon-check-circle"></i>
      <div data-test="avatarMsg" class="c-inner-text">Avatar updated!</div>
  </div>

The Cypress is the following:

  it('user1 changes avatar and profile settings', () => {
    const profilePicture = 'imageTest.png' // at fixtures/imageTest

    cy.getByDT('settingsBtn').click()

    cy.fixture(profilePicture, 'base64').then(fileContent => {
      cy.get('[data-test="avatar"]').upload({ fileContent, fileName: profilePicture, mimeType: 'image/png' }, { subjectType: 'input' })
    })

    cy.getByDT('avatarMsg').should('contain', 'Avatar updated!')
  });

Versions

• Cypress 3.5 (working on Chrome 79)
• Cypress 4.0.1 (not working on Firefox)
• Firefox Developer Edition 73
• Mac OS Mojave