Simple: just declare the method appropriately:
string documentProvider(ProbablyAStringButIDontKnowForSure message, List<string> filenames
{
...
}
Or
string documentProvider(ProbablyAStringButIDontKnowForSure message, IEnumerable<string> filenames
{
...
}
And then call it:
List<string> filenames = new List<string>();
...
string result = documentProvider(myMessage, filenames);