I am the CTO of an organization and we basically use Microsoft Teams for all of our internal conversations. Something that I've noticed is that we upload an image (*.png or *.jpg) to a channel, if you do the following from the iOS application, it will treat the file as an Outlook attachment rather than the original file type:
Click on the image
Click on the download icon
Click on one of the actions (message, mail, save to files, etc.)
If we download the file from the desktop application, then it will download as the original file type.