I set Stroke Style to Miter Join, but that didn't work.
Any solution for this?
You can do this in 2 ways.
An easy way to do this is to duplicate your shape and put it in the background (menu object > transform > move > copy 0-0-0
Then apply a thick stroke the color and thickness you want on the shape in the background. They must have the exact same size and position, so use the copy command mentioned above.
The shape on the foreground will keep its shape and the shape on the background will have proportional mitre.
This method works better with some script text styles or some complex paths.
Use the same shape and adjust the stroke, mitre and alignment of the stroke to be "outside."
This doesn't always work well with complicated paths or text. The first method is easier for this reason in some situations.
For the non-believers, here's why method 2 isn't always good unless the vectors are merged together (which isn't always ideal when editing.)