Fix boundary value extraction for form-data requests

[michaelwolz]

The changes introduced in #7306 fixed several bugs with multipart/form-data requests on iOS. However, the extraction of the actual boundary value might fail due to the value being surrounded by double quotes, which is allowed and happens occasionally (See MDN Reference which utilizes double quotes for the boundary in their example: https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST#example).

In addition, the Content-Type header might include other keys such as charset.

This change extracts the boundary value regardless of the order of the individual keys within the Content-Type header.

Example Code

(Assuming that the plugin override is enable in capacitor.config.ts)

const body = new FormData();
body.append('key', 'value');

await fetch(url, { method: 'POST', body, 
  headers: {
    'Content-Type': 'multipart/form-data; boundary="boundary"; charset=utf8'
    //  'Content-Type': 'multipart/form-data; boundary="boundary"'
  }
});

The current implementation only considers the last string after any equal sign (contentType.components(separatedBy: "=").last). This results in --utf8 being set as the boundary in the request body. Even if there is no other key involved in the request it will include double quotes " in the request body, leading to a malformed request.

Working example including the changes can be found here:
https://github.com/michaelwolz/multipart-form-data-not-working-on-ios

[michaelwolz]

Actually I just saw that the same issue exists with Android. I will try to also provide a fix for this :).

Edit: Done in bc6a6e0, example app also updated to include android platform

[michaelwolz]

Note that this part was missing in the iOS implementation but present for Android already.