I should have looked a little harder at the forum you were posting to.
I'm not using company webmail - so I can't say for sure. But having my own email (comcast) within the "mail" app - I've noticed that ANY attachments will sometimes fail to download in email via Edge. Seems a lot more reliable with a WIFI connection, and more trouble prone with larger attachments.
I've opened an excel sheet - but could not edit anything, just read-only. I cannot recall if it was an excel sheet, or an excel sheet embedded within a word file. So I'm not sure that is any help.
Last thing - you say webmail - as in access via Safari? If that be the case, are you able to download attachments? I've never been able to download anything in Safari outside of PDFs or pictures. (and by download I mean read within Safari - not save to a particular directory) I "think" the email App on the iphone is what actually supports the word and excel file viewing - which could be why you can't via Safari / webmail.
Take all that with a grain of salt - I'm not an expert on this.