Hi Tom,
Attachments will be added through ESS by an employee, and only he is authorized to edit or change the attachments. Once it goes to the approver, he can view it and approve the expense report.
All file formats except zipped or compressed files are acceptable.As you are adding from the ESS or TRIP in backend sap system, so the functionality is same.
Approve travel expense should behave like SAP Inbox and all attachments type, which opens in inbox should open in fiori as well.
Regarding word documents issue with iphone,there are many tools in appstore, like documents-free and other cloud versions like, upload the same in google docs and view it.if you are ready to pay some then iWork
BR
Tejas :)