What?

This PR ensures that any valid image uploaded to the Gallery block within the mobile apps is correctly marked as valid.

Why?

Following the updates to the Gallery block's error notices in #43946, the following error began displaying when uploading images to an existing Gallery block: If uploading to a gallery all files need to be image formats

Note, the previous implementation of the error notices had not been supported on mobile. The referenced PR exposed a previously 'silent' issue by introducing an error notice that worked within the apps.

How?

The error began firing as the isValidFileType function was returning false for existing images in a Gallery block on native:

The above function works for newly uploaded images, as those images return a file object similar to the following:

{"alt":"","caption":"","id":3373,"title":"hello-world.png","type":"image","url":"https://example.com/2022/10/hello-world.png"}

However, on native, images that are already uploaded to a Gallery block return a file object similar to the following:

{"clientId":"45c8e5a2-d20d-4d77-a224-88c426e93878","id":2255,"url":"https://example.com/2022/10/hello-world.jpg?w=473","attributes":{"url":"https://example.com/2022/10/hello-world.jpg?w=473","alt":"","caption":"","id":2255,"link":"https://example.com/hello-world/","fullUrl":"https://example.com/2022/10/hello-world.jpg","linkDestination":"none","sizeSlug":"large"},"fromSavedContent":false}

To account for the different data that's retrieved for already-uploaded images on native, the isValidFileType function has been updated to gather the media_type details from the file's raw image data:

Testing Instructions

1. Navigate to the editor in the iOS or Android app.
2. Add a Gallery block with multiple images and save the post.
3. Tap the appender to add a new image to the block.
4. Try a few different methods for adding the image, such as from your device or from your site's Media Library.
5. Confirm that the If uploading to a gallery all files need to be image formats doesn't fire unexpectedly after uploading valid images.

Screenshots or screencast

From these screenshots taken immediately after uploading an image to the gallery, you'll see that the first screenshot shows an error message despite the fact a valid image had been uploaded:

Before	After