I recently noticed a bug associated with the file upload widget and excel reader node. The excel reader is unable to read files from a file upload widget when there are spaces in the filename. I would receive the error seen in the image below. Upon removing the spaces the excel reader was able to read in the file no problem.
Yes, this is a very annoying bug. We also already stumbled across it and the developers already found a fix (internal reference AP-16152). This will be part of the upcoming bug fix release. I will let you know whenever it is available.