See issue description in parent issue.
Basically, we should never send the mimeType when saving a BinaryFile field, it's up to server to correctly compute this value from what it receives.
Most likely the same happens with Image and Media fields as well.
Steps to reproduce:
- Create a content having a File field type.
- Add a file (let say an image file)
- Publish the content
- Edit the content
- Click on replace this file and select an different type of file (let say an audio file)
- Open the dev tool console of your favorite browser to check the request that will be sent
- Click on save.
- Check the PATCH request, for the image fieldtype the mime type will still be the previous one (so an image in our example).