feat: show error upon FileReader failure

[mantariksh]

Problem

We have a high frequency of "Required but no attachment content" validation errors from attachment fields, meaning the user and the frontend think that the attachment was uploaded successfully, but the backend discovers that the attachment content is actually missing. These errors occur almost exclusively on Android phones.

Closes #23

Solution

The error occurs when the user attempts to upload a file directly from Google Drive. The UI behaves as if the file has been uploaded correctly, but in fact FileReader has failed silently because we have not set an error handler on it.

The ideal solution would be to fix file uploads from Google Drive, but this is unlikely to be straightforward and likely to introduce a lot of special cases in the code since Google Drive files clearly behave differently from files in the local file system.

Hence this solution does two things:

1. Adds an error handler which prompts the user to download their file first if they are using Google Drive.
2. Only shows that the file was uploaded successfully if the file was saved as a blob successfully. This makes sense because the upload is really only successful after we have set vm.field.file. It also prevents situations where the UI first behaves as if the attachment was successful, then suddenly flips to an error when FileReader.onerror is called.

I also took the opportunity to introduce a few small refactors to simplify the attachment component.

Screenshots

Tests

• Upload an attachment from Google Drive on an Android phone. The error should show as per the screenshot.
• On both desktop and mobile, upload a valid attachment. The UI should behave correctly. Make sure that this works for .zip files.
• On both desktop and mobile, upload an attachment which exceeds the maximum allowed size. You should see the correct error message, and you should subsequently be able to upload a valid attachment and submit the form.
• On both desktop and mobile, upload an attachment of an invalid file type (e.g. .js). You should see the correct error message, and you should subsequently be able to upload a valid attachment and submit the form.

[mantariksh]

@syan-syan seeking your approval on the error wording pls

[liangyuanruo]

i made some minor suggestions, lgtm otherwise!

[r00dgirl]

@mantariksh sorry late but its ok!