Crash when clicking "upload"

[joserc87]

Steps to reproduce:

• Go to upload.farm
• without clicking on "choose file", just go ahead and click "upload" resulting in a 502 Bad Gateway

Expected behaviour:

• Ideally the select file + upload would be one button but that requires more work
• Instead we could show a message to the user
• Or at the bare minimum, just don't do anything but don't crash

I open this just to keep track of it as I encountered it while working on something else. I will send a pull request if nobody fixes it sooner.