file upload option disappearing in teams bot

We have Teams bot (we're the developer).

When users first start teams, they have the file upload option available under the + button next to the type a message box.

However, shortly afterwards, this option disappears. Not all users, just some. Seems to occur for both Mac and Windows users.

Our manifest looks like this:

{
    "$schema": "https://developer.microsoft.com/en-us/json-schemas/teams/v1.19/MicrosoftTeams.schema.json",
    "manifestVersion": "1.19",
    "version": "1.0.7",
    "id": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
    "developer": {
        "name": "example",
        "websiteUrl": "https://www.example.com",
        "privacyUrl": "https://www.example.com/privacy",
        "termsOfUseUrl": "https://www.example.com/privacy"
    },
    "icons": {
        "outline": "example_mono.png",
        "color": "example_portrait.png"
    },
    "name": {
        "short": "example",
        "full": "example, your digital assistant"
    },
    "description": {
        "short": "example is a digital AI assistant designed to streamline your workflow.",
        "full": "example is an advanced AI-powered professional assistant developed by example, crafted to enhance productivity and efficiency in the workplace. With a focus on accuracy and a professional demeanor, example assists with information retrieval, scheduling, communication, and various tasks, ensuring that you have more time to focus on what matters most in your business."
    },
    "accentColor": "#fb9745",
    "bots": [
        {
            "botId": "yyyyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy",
            "scopes": [
                "personal"
            ],
            "isNotificationOnly": false,
            "supportsFiles": true,
            "commandLists": [
                {
                    "scopes": [
                        "personal",
                        "team"
                    ],
                    "commands": [
                        {
                            "title": "New",
                            "description": "Starts a new conversation"
                        }
                    ]
                }
            ]
        }
    ],
    "permissions": [
        "identity",
        "messageTeamMembers"
    ],
    "webApplicationInfo": {
        "id": "yyyyyyyy-yyyy-yyyy-yyyy-yyyyyyyyyyyy",
        "resource": "https://RscBasedStoreApp"
    },
    "validDomains": [
        "*.ngrok-free.app",
        "*.ngrok.app",
        "exampledev.ngrok.app"
    ]
}
```

As far as I can see from the current manifest schema, the bot needs no other scopes.

Is this a known bug or are we screwing something up with the config? Any help appreciated!