Pasting an image from the clipboard on iOS can insert multiple images

When pasting an image from the clipboard on iOS 13 that has multiple representations, it can insert all representations instead of just a single image. It may affect earlier versions of iOS, but I haven't seen it until iOS 13.

##### Steps to Reproduce

Here are two methods:

1. Go to trix-editor.org in mobile Safari on iOS 13
2. Focus the field and insert a Memoji from the keyboard
3. It will insert a JPG and PNG

OR...

1. Go to trix-editor.org in mobile Safari on iOS 13
2. Take a screenshot and tap the floating thumbnail
3. From the screenshot markup screen, tap the share button and choose "Copy".
4. You'll now have a JPG and PNG on the clipboard.
5. Focus the editor and choose "Paste"
6. It will insert both the JPG and PNG

Here's an input inspector profile of pasting the screenshot: https://input-inspector.now.sh/profiles/F4Rcp0EGKqE6OJcDFL1L

##### Details

* Trix version: 1.2.2
* Browser name and version: Safari
* Operating system: iOS 13.3

No source file or test is named. First reproduce the behavior at trix-editor.org in mobile Safari on iOS 13 using the Memoji or screenshot-copy steps, then trace the editor's paste handling; done means a clipboard containing multiple image representations inserts only one image.

Written by the indexing model from the issue text.