Allow me to be merciful and give you an actual explanation of why your software would be completely ineffectual.
Ransomware can and (according to Murphy's law) will encrypt any file that the malware has access to. Software running on a compromised computer to protect files would be useless since the attacker would have access to the computer anyway, and even if they didn't have read/write access because of your program, they could simply kill the task and go on jacking up your data.
Additionally, protecting only certain file-types is even more absurd. First of all, although you might be able to obfuscate document files (security through obscurity, oh my!) to hide from a static attack (i.e. amateurish program that encrypts all files ending in .docx), any legitimate threat will sail right by software-implemented protection/obfuscation. Not to mention the difficulties your program would have in verifying that a .docx file is actually a .docx file and so on so forth for every filetype.
Instead, here's the real solution: lock down your environment with proven techniques like firewalls, OS updates, and physical security, and protect every single file, because white-listing what files you want to be secure when you expect a hacker has compromised every other file on the system is absurd.