how do you change .blender files to a file compatable with unity?

i am making some wepaons on blender and transfering it to unity to put out on the unity store, but when i try to upload, it gives me an error saying that blender files are not allowed. i did at first try to get rid of the blender files, but when i did any model that has the parts was automatically missing, making the model useless. is there anything that can convert a blender file to a compatable unity file, and preferably free?

“All models that you submit must be in FBX or OBJ format.”
Blender can export to these formats.

More: http://unity3d.com/asset-store/docs/submission-guidelines.html

Export it as a FBX