Ya, this is a fairly common issue.
FTP Task only knows how to process 1 file, and 1 file only the *_Filename.xls your trying won't work.
You need to create a variable to hold the name of the file. Since the file changes and it is dynamic you have to store the name in the variable.
Then configure the ftp task to get the name of the file from the variable.
On your ftp task make sure to set the Delay Validation to True.
The only step left will be to figure out how to populate the variable with the correct name of the file.
Do you know what the first 5 characters of the file name will be each month?
Will there be 1 or many files each month?