Provide better customizations for custom file picker implementation

[szuperaz]

On mobile OS-es it's common to use a custom file picker implementation. This requires two customization points:

• Provide a hook to create a custom file upload button: it's already supported ✅ docs
• Make sure files selected from the custom picker can be uploaded using the attachmentService.filesSelected method ❌ we haven't tested this in detail, but possibly we need to do some work around this
• For the best DX we should also a create a guide for this