ZIP imports fail on uncommon filetypes

In Zendesk 9788, a user reported this error being displayed when they tried to import a ZIP created using the export function originally discussed in #304

Parameter validation failed: Invalid type for parameter ContentType, value: None, type: , valid types:

It turns out that the python mimetypes library on our production system fails to guess the type of .webp and .avif files, among others. There's an asymmetry in that, when uploaded, the browser reports the mime type, so users of the editor can introduce these files without a hitch, but the import process doesn't know what the mimetype was.

The workaround, in this case, was to remove the problem files from the ZIP, import the rest of the archive, and then use the editor to re-upload the single .webp file which was actually being used (the rest were not used in the storymap)

A partial solution would be to configure mimetypes to understand more filetypes. .webp seems pretty natural. I was unfamiliar with .avif but CanIUse reports that most browsers except Edge support it. Still, this kind of kicks the can down the road without necessarily solving it.

Another option, which may not actually be possible, would be to write the mime types into the manifest.json included in the export ZIP, if we can recover them, perhaps from boto. Using this instead of "guessing" should ensure that any file we've accepted through the editor can also be accepted through the import.

See also #487 which suggests limiting which files are included in exports. In this case it would have filtered out most, but not all, of the uncommon filetypes.

Start by tracing the ZIP import path and how manifest.json and Python's mimetypes library determine ContentType. Compare that with the browser upload path for uncommon types such as .webp and .avif. Done means ZIP imports handle accepted files without the reported validation error, with coverage for the affected filetypes.