.M File

File extension m is associated with the Objective-C, a general-purpose, object-oriented programming language based on Smalltalk language developed by Apple, Inc

This type m file type represents scripts or sources written in Objective-C.

How to open:

Use Apple Xcode or any IDE or text editor.

How to convert:

Source files usually cannot be converted, but you can find parsers that may be able to translate it to other programming language.