Why can’t I upload an attachment in the iOS app?
For the time being, the only type of attachment we’re able to add in our iOS app are image files. Adding an attachment to an email in iOS presents a bit of a problem, because unlike Android devices or desktop computers, iOS does not contain a user-accessible file storage system (except the photos folder). This means that there's no location from which we can upload an attachment, nor one to which we can save a document. You'll notice that other mail apps such as Apple Mail or Gmail do allow you to attach other file types, but only from other third-party apps such as Google Drive or Dropbox; the only files they're able to upload directly from your device are photos.We do hope to be able to support these types of third-party integrations in the future, but don't right now. Our apologies for any inconvenience that causes.