A 12" miter saw blade of the ilk normally found on a miter saw or cut-off saw looks like this:
The teeth on the saw blade are carbide material that is very hard. You will not want to take a file to those teeth as they will very quickly kill your file.
This type of saw blade has gained huge popularity in recent years because the hardness of the carbide material helps the saw to keep an edge for a long time. Another attribute of this type of saw blade is the extremely smooth cut that can be achieved with them. This smooth cut is possible because of the precision grinding of the carbide tips at the time of manufacture.
I do not recommend hand sharpening this type of blade because of the distinct possibility that hand sharpening will result in tooth shapes that are not perfectly uniform all the way around the blade. Variation of tooth shape, length, edge angles, and width will have a direct effect on how smooth and nice of cut that can be produced by the blade.
So find a seasoned professional saw resharpener that has the necessary power grinder equipment that can result in an almost factory equivalent sharpening job on a carbide tipped saw blade. Most blades of this type can be resharpened at least once or twice provided there are no teeth that are chipped, broken or missing.