I would suggest to store it in a string attribute and use custom format checking and formatting.
Perhaps using a custom widget with own formatting for displaying.
It seems that out of the box Mendix doesn't support this.
Hope this helps.
EDIT: For calculation of the decimals you can check the following example: https://modelshare.mendix.com/models/8ef7ad5c-de6c-4241-b36f-d1808783ce6c/calculationofdecimals
The outcome for the provided formula is: