how to upload an osm file to this website

edit

You can't. But you can do the following:

1. Upload it somewhere else (Dropbox, box, google drive, wetransfer) and provide the link
2. You can also upload it on the UnmetHours slack channel and provide the link
3. You can trick the website by changing the file extension to *.jpg to be able to upload it, and provide the link to users specifying they need to change the extension back to *.osm.
4. If not too big you can paste the text content of the .osm file to a website like Pastebin

I'd avoid 3. personally as I find it somewhat confusing and not user friendly at all.