Displaying entries from some categories but different channels

I have a channel called “offices” with a limited number of entries. Every entry is assigned to a different category from a Category Group called “office-categories”. For instance the first entry is assigned to a category called “office-1”. I have another channel called “files” with unlimited entries assigned to categories from the same Category Group “office-categories”. In time a lot of entries will be assigned to the same category “office-1”. I cannot use the Relationship in the first channel because there are so many entries in the second channel that make difficult to find the ones that belong to a specific category. In the single-entry page of the first channel I like to have the details of the first channel entry and all the entries (all the files) from the second channel that belongs to the same category “office-1”. Can someone tell me if this can be done in EE and how? Thanks a lot