If you encounter this error when trying to download or open a Receipt in an Expense reports:
You'll want to replace a file on the server.
On your server you'll want to go to a path to your portal folder that should be similar to this:
Within this folder will be an OpenFile.aspx file, you'll want to rename the end to be .bak, and then place the attached OpenFile.aspx in its place.
After this, you should be able to return to your Expense Report and view the receipts properly!