g++ only looks for .lib files

I am using Windows 10 with mingw-w64. I tried compiling a program with a statically linked library. I used this command: g++ main.cpp -Llibs/ -lfoo. But MinGW says it can't find the library files, so I tried renaming foo.a to foo.lib and voila, the compiler found foo.lib. Why doesn't MinGW see *.a files anymore?