It’s quite strange but Unity can’t show your enum as bitmask / flags out of the box.
Fortunately this can be fixed fast and easy.
First of all we need an attribute which should inform Unity that we want it to show our enum as flags and not as usual one-option list of enums. That’s pretty simple and straightforward.
Next part is a custom
PropertyDrawer. Its purpose is to draw a property with which
EnumFlagAttribute was associated.
Nothing special as well. Maybe except the 11-th line where we receive the value of enum from
SerializedProperty. Unity’s documentation lacks detailed information about
PropertyDrawer.fieldInfo but that’s one of the ways how to use it.
Usage is easy peasy. Just associate
EnumFlag attribute with an enum field and enjoy your bitmask in the Inspector.