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.