I can reproduce the problem fairly consistently with these steps
1. select and add the file
2. if it shows up under the message text box, the remove it and go back to 1
Just repeat that loop for a few times, the longest it took for me was 3 times. file size does not matter.
when the file does not show up, I have tried a few things
- press the refresh circle on the browser to refresh the page with no effect
- waiting for a long time, minimize and maximize the browser, switching browser tabs has no effect
- if you don't see the file below the message text box and then send it, then you will see the file in the sent message