iOS app - treat DNG files as attachment instead of images

[mattermod]

With reference to https://github.com/mattermost/mattermost/issues/25652 currently the iOS app considers DNG files (iOS raw image format) as an image. This causes the file to be uploaded as image but since non Apple devices cannot render DNG files by default, it ends up looking like a broken image.

We need to update the logic in mobile app to consider DNG files as attachments instead of image. A nice to have would be to consider anything other than common image formats ('png', 'jpg', 'jpeg', 'bmp', 'tiff', 'svg', 'xcf') as an attachment instead of an image.

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JIRA: https://mattermost.atlassian.net/browse/MM-58145

[apshada]

@marianunez I would like to work on this issue

[marianunez]

Thanks @apshada! All yours 👍