binary_filesize: This field is required. when "reusing" the upload form

[Daniel Manrique]

How to reproduce:

1- Click on the "New snap" link in the top right corner
2- Ignore the name registration form and click on "proceed directly to upload"
3- Click on the "no file selected, click to upload a file" uploader.
4- Upload a non-snap; e.g. a text file or an image. I used a .png.
5- Click on submit to the store.
6- (You'll get "__all__: The upload does not appear to be a valid package." and be left in the same page).
7- Click on the uploader component again.
8- Select a valid snap file
9- Click on "submit to the store"

Expected result:
- Upload accepted or at least processed returning meaningful errors like the name is not registered

Actual result:

- Upload not accepted
- Red message banner at the top: binary_filesize: This field is required.

The initial report for this mentions the error message is cryptic and not easily actionable by the developer. But it'd be better if the uploader widget reset properly and didn't send that error in the first place.

Initially reported here: https://forum.snapcraft.io/t/reviews-and-other-trivial-user-experience-issues-at-the-store/2031/12

[William Grant (wgrant)]

What upload form.