3817101 - Why is my emailed receipt to Concur not being stored .P7S file?

I am trying to email a receipt to receipts@expenseit.com and receiving a bounce back email stating: 

Your emailed receipt to Concur could not be stored
Hi (user name), The attachment for file named ".p7s" appears to be an invalid file type and could not be stored by Receipt Store. If the file is a receipt image, upload it directly from the expense entry to display details of the error.

Thanks for choosing us,
The ExpenseIt Team

User is trying to email a receipt but is getting the Your emailed receipt to Concur could not be stored. p7s appears to be an invalid file type. , KBA , CON-EXP-EXP , Expenses , Problem