Comment 0 for bug 1715997

How to reproduce:

1- Select a snap and click on "Upload new version" (top left)
2- On the upload form, fill in the support url field if present (any valid url will do).
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 snap
9- Click on "submit to the store"

Expected result:
- Upload accepted

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