The form to submit an asset should not be cleared if there is an error

[Zylann]

On Firefox

I've tried to submit an asset, but I didn't realized the Link to thumbnail (To be removed) really had to be filled by something. So I got an "Error, thumbnail must be a string". I clicked "previous" to go back to the form, but it was empty. I had to refill it entirely, twice, which is cumbersome just for an error I got because of one field. I though Firefox would remember these, but apparently it didn't... Is there a way for the fields to not be cleared?

[Calinou]

We should also make use of HTML5 validation to prevent submitting invalid forms.