"Personnel" is a mass noun--also called an "uncountable noun." It refers to a labor force: the people who staff an office or a company. If a company only has one employee, he can still be referred to as "our personnel," but it's usually used to refer to a group of employees.
"Personals" is a word popularized by newspapers. It's a contraction of "personal advertisements" -- announcements people place with the goal of finding someone. Usually a stranger to meet for the potential of romance, but it could also be someone searching for a friend or family member with whom they've lost touch.
"Personal" means, literally, "pertaining to a person." When used in a phrase like "personal files," in the context of a workplace, it refers to files that a worker keeps at his desk, but only for personal use (his friends' phone numbers, the dates of his family members' birthdays, etc.)--as opposed to the much larger quantity of files of company business that he uses to perform his job.
A personnel file is a file maintained by the company on each individual worker. As noted, this is where complaints about a worker's bad habits would be stored, but presumably there will also be information about his successes, as well as accounting information such as his age, salary, number of years in the company, etc.
"Personnel" is simply the French word for "personal." Both words were derived from Latin personale. As I've mentioned before, England was conquered by France in 1066, so for several centuries French was the language of commerce, government and scholarship. This is why there are so many French words in our language.
The English people never fought a revolution to overthrow the French domination. On the contrary, the French people in England slowly gave up much of their French culture and assimilated--this included speaking English instead of French.