File Dependency Resolver

You are given a collection of files and a list of dependency pairs representing the import relationship between these files. Each dependency is expressed as a pair (A, B), which means that file A depends on file B (i.e., file B must be processed before file A).

Your task is to write a function that takes the list of file names and the list of dependency pairs, and returns a valid processing order of the files such that each file appears after all the files it depends on.

If no valid ordering exists (due to circular dependencies, for instance), your function should detect this situation and report an error or return an appropriate indicator.

Assume the following:

• The list of files is provided as an array of unique strings.
• The dependency list is provided as an array of pairs or tuples, where each pair contains two strings from the file list.

Consider the efficiency of your solution for large inputs.

Write your solution in a programming language of your choice.