Filch's office is a small room with a single oil lamping hanging from the ceiling. It smells of fried fish. In it are filing cabinets with details of the misdeeds of students. Fred and George have an entire drawer to themselves.

In Filch's office you'll find:

filing cabinet with records of the misdeeds of students, including an entire drawer for the Weasley twins (CS cool
chains and manacles (not used these days but kept polished just in case) (CS cool
cabinet full of confiscated magic items marked "Confiscated and Highly Dangerous." (PA10)
desk and chairs
lots of forms to write up malefactors
probably on the ground floor because Filch took Harry "back downstairs" to write him up for befouling the castle, and because the Vanishing Cabinet which Peeves broke directly over Filch's office (CS cool was located on the first floor (OP)