These files are any files that Windows is able to open — images, MS Word documents, MS Excel spreadsheets, Adobe PDFs, scanned documents, photographs, virtually ANYTHING.
For example, many of our users scan and store both sides of every patient’s insurance cards in their library.
One of the listed files in the library is a “Patient Photo”. Let’s open that file:

Those library files might be scanned handwritten chart notes, test results reports from outside testing agencies, letters to/from patients, doctors, lawyers, subpoenas, or treatment plans.
The possibilities are literally endless.