There are multiple options to create REST APIs to expose Salesforce data and processes, some of the popular ones being -
If you always wanted to learn salesforce.com APIs and what you could do with them, but too afraid to ask.
JSON Web Token, or JWT, is a way to enable authentication and get you access to a resource.
Salesforce changed the way files are managed with Lightning Spring 17 release, and this introduced a more elaborate procedure to reach the files from external systems.
Rest APIs are quite easy to use and my preferred way to get things done.