Development / upload wrapper helper documentation

[AdSchellevis]

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• [x] I have read the contributing guide lines at https://github.com/opnsense/docs/blob/master/CONTRIBUTING.md
• [x] I have searched the existing issues and I am convinced that mine is new.

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Add documentation for new SimpleFileUploadDlg() and it's related controller methods.

• https://github.com/opnsense/core/commit/d66502881bc743f5492320617737711d4c42e552
• https://github.com/opnsense/core/commit/6f87280cd406693579004717e71ec480759cd538
• https://github.com/opnsense/core/commit/a8d1a84fbae49e148c9466a29f9d1ed5ac56c23c (export to csv)