C# logic use only file selected by user

In a C# 2010 desktop application, I want to change the application so that it will only allow users to select a
report that meets the following criteria:
1. part of the file name is "ErrorReport" and
2. The last node of the file name is .xlsx or .xls.

So far I have the following code:

string[] excelFiles = Directory.GetFiles(strDirectoryLoc, "*ErrorReport*")
                                         .Select(path => Path.GetFileName(path))
                                         .Where(x => (x.EndsWith(".xlsx") || x.EndsWith(".xls"))
                                                   && (!x.StartsWith("~")))
                                         .ToArray();

This code works when I am selecting only excel (*.xls or *.xlsx) files.

The problem occurs if the user selects a .pdf file iniitally and there is actually a file in the directory path
that meets the criteria I listed above. The code will ignore the .pdf file the user selects and will actually use the
excel file that is in the directory path.

Thus how can I change the code listed above to say the .pdf file is invalid?