Offline: Image Attachment/Attachment to show/download content after attaching

[ebruchez]

Following #5207.

Attachments are saved, however the Image Attachment control cannot show an image that has been attached, both:

1. just after having attached, which means that we have a File object at our disposal (upload: scheme)
2. after saving the data, which means that we have no File but a path to the persistence layer

In online mode, an <img> HTML object requests data to the XForms servers, which then proxies this appropriately to either the temporary file or the data from the persistence layer. In offline we do not have a server that can do this kind of proxying.

• figure out mechanism whereby we can create the image object
• fix Image Attachment
• also fix Download Attachment
  • see also fr:attachment: use download attribute #6198

[ebruchez]

When we have a File object, we can use URL.createObjectURL(). This should cover showing the image and downloading the file (with <a download>?).

We could probably make this work even after saving the data.

However, this won't work if the form is closed and reopened, as in that case we have a ReadableStream and that cannot be transformed into a URL. That could be an acceptable limitation as a first step, but not ideal in general.

However:

• Attachments: we could disable the Download link in such cases.
• Images: we could read them and transform them into a Blob if they are not too large. After all, they will have to be uncompressed and shown in the browser eventually.
• Videos: not needed right now; could MediaSource help? For smaller videos, we could do what we do for images. But videos tend to be be much larger than images, in the order of GBs vs. MBs.

[ebruchez]

Changed title to indicate that the data retrieval now works for attachments in the same session where they have been attached. We don't yet support reading such attachments from persistence in a new form session.