You’re probably not doing anything wrong. There are restrictions on uploading files until you’ve made a few posts on the forum.
To upload an image or .skp file, click the 7th icon from the left, above where you type your replies:
Or just drag the file into your reply…
When typing a reply on a computer, you see the ![Upload… blah in a text field at the left and on the right you see a preview of what it will look like once you’ve posted it including images. On your phone, click the 1st icon on the right that looks like a screen(not shown in my image above) to see a preview of your post with images. Then click “hide preview” at the bottom right to go back to typing.