How to upload images and file to a server in Flutter?

Your workaround should work; many servers will accept application/x-www-form-urlencoded as an alternative (although data is encoded moderately inefficiently).

However, it is possible to use dart:http to do this. Instead of using http.post, you’ll want to use a http.MultipartFile object.

From the dart documentation:

var request = new http.MultipartRequest("POST", url);
request.fields['user'] = '[email protected]';
request.files.add(http.MultipartFile.fromPath(
    'package',
    'build/package.tar.gz',
    contentType: new MediaType('application', 'x-tar'),
));
request.send().then((response) {
  if (response.statusCode == 200) print("Uploaded!");
});