use @primary_extension_index in find_by_filename
change MUMPS' primary extension from .m to .mumps

.m currently clashes with Objective-C, and MUMPS is chosen before
Objective-C when creating .m files in Gist. Even if .mumps isn't a
legitimate MUMPS extension, I think we should change it anyway since
Objective-C is far more likely to be intended when a user uses .m
add .m as an alternate extension for MUMPS so it's still picked up