Using wildcards wont be a reliable one, in your case U8319716 this part of file name may also change.
So maintain a separate folder especially for these files, then get all the files inside the folder using the below vb.net expresion.
arrayOfFilePath = Directory.GetFiles("folder path","*.xls")
This expression will return array of file path which will be string, this function will only get all the .xls files from the folder.
This array you can pass to for each and read one by one. Now there won’t be any issues when file name changes.
Further you can refine the function using something like this,
arrayOfFilePath = Directory.GetFiles("folder path","U8319716*_*.xls")