You need specify custom files types when creating the PickOptions and they can be customized per platform.
Per docs , PickOptions.FileTypes has many types in each platform,
On Android and iOS the files not matching this list is only displayed grayed out. When the array is null or empty, all file types can be selected while picking. The contents of this array is platform specific; every platform has its own way to specify the file types. On Android you can specify one or more MIME types, e.g. "image/png"; also wild card characters can be used, e.g. "image/*". On iOS you can specify UTType constants, e.g. UTType.Image. On UWP, specify a list of extensions, like this: ".jpg", ".png".
You can refer to the following sample code:
I add a button to trigger the selection event,

private void PickFileButton_Clicked(object sender, EventArgs e)
{
    Task<FileResult> results = PickAndShow();
}

//for custom types
async Task<FileResult> PickAndShow()
{
    var customFileType = new FilePickerFileType(new Dictionary<DevicePlatform, IEnumerable<string>>
    { 
        { DevicePlatform.iOS, new[] { "com.adobe.pdf", "UTType.Image" } }, // or general UTType values
        { DevicePlatform.Android, new[] { "application/pdf", "image/*" } },
        { DevicePlatform.UWP, new[] { ".pdf", ".jpg", ".png" } },
        { DevicePlatform.Tizen, new[] { "*/*" , ".png" } },
        { DevicePlatform.macOS, new[] { "pdf" , "public.image" } },
    });
    
    try
    {
        var result = await FilePicker.PickAsync(new PickOptions
        {
            PickerTitle = "Please select files",
            FileTypes = customFileType,
        });
        if (result != null)
        {
            FileName.Text = $"File Name: {result.FileName}";
            if (result.FileName.EndsWith("jpg", StringComparison.OrdinalIgnoreCase) ||
                result.FileName.EndsWith("png", StringComparison.OrdinalIgnoreCase))
            {
                var stream = await result.OpenReadAsync();
                FileImage.Source = ImageSource.FromStream(() => stream);
            }
        }
        return result;
    }
    catch (Exception ex)
    {
        // The user canceled or something went wrong
    }
    return null;
}

Note:Don't forget to add WRITE_EXTERNAL_STORAGE and RAED_EXTERNAL_STORAGE in your manifest file.
Official reference docs: https://learn.microsoft.com/en-us/xamarin/essentials/file-picker?context=xamarin%2Fxamarin-forms&tabs=android