if i set validation rules for the asset filed type like this

  image_gallery:
    type: assets
    validate: required|image|mimes:jpeg,bmp,png
    display: Image Gallery
    instructions:  Choose or upload an image for your Project, You can select multiple images up to 5.
    container: portfolio
    folder: /
    max_files: 5
    restrict: true
    destination: assets/projects
    mode: grid

it keeps me giving error The Image Gallery field must be an image.,The Image Gallery field must be a file of type: jpeg, bmp, png. though i have select an image and it is png image

please help me how to setup the validation on asset fieldtype, like image type or may be image min width or min height etc.