I've just checked that '
Attach files' facility fairly extensively, and it seems to be working fine, at least for me, so maybe there's an issue on your end.
That said, once you've used the '
Attach files' facility to go to your device and select the file/pic you want, then you've '
opened' your chosen pic/image, what exactly happens? Anything at all, or what? Do you even get that far?

Does it bring you back to the Forum software and the Post reply text entry screen, and do you get the '
Attachment' file showing in the '
Attach files' block below the text field, with the word '
Insert' showing in its top left corner, so that you can click on '
Insert' and then '
Thumbnail'? For more, see notes 1 & 2 below:
1. If you do get all that, have you placed & clicked so that your cursor is in the text field where you want the pic to appear, or have you forgotten to do that so the software doesn't know where to insert the thumbnail? If that's the case, it won't upload and attach the image, or if it does, it stuffs it somewhere on the Forum servers I've never found/been able to find, but your pic should still appear as a linked file with the word 'Attachment' showing above the pic if you go on and submit the post; in which case you should then be able to edit your post and follow the numbered steps to finish the 'Insert as a thumbnail' as listed in the 'PLEASE UPLOAD...' Sticky.
2. If you don't get that, you might want to check that when you click on 'Attach files' and it takes you to your devices' file manager, that the file manager hasn't defaulted to only show 'Compressed (zipped) folders' or files - it needs to show All file types, or at least JPEG & WEBP files - there might be others that it'll accept, but I cannot recall any more off the top of my head right now.
Over to you.