ENH leverage FormData to support file upload

[lekoala]

Description

Currently, ajax forms are submitted using serializeArray. This doesn't allow sending more advanced content, like a file upload

Manual testing steps

• Add a FileField in the cms
• Check that $_FILES is empty on save

With this change, files are submitted properly. It also offers the opportunity to do more advanced things (eg: sending binary canvas data as a blob) and, all things considered, should probably work better because it uses a native js feature instead of jquery serializeArray

Issues

#1852

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