You can't. But you can do the following:
- Upload it somewhere else (Dropbox, box, google drive, wetransfer) and provide the link
- You can also upload it on the UnmetHours slack channel and provide the link
- 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.
- 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.
