Cannot attach certain file types to email in iOS

Estonia uses Digidoc format as national standard for signed electronic documents. This signature is legally binding and legally equivalent to hand written signature. The container file containing the file(s) and signature in a *.bdoc file cannot be attached to e-mails from iOS as the bdoc file is visible as grayed out while browsing for files to attach. This bdoc file is the official file type that state institutions are legally bound to accept and that citizens are required to send when doing official correspondence. There is a Digidoc browser app for iOS and one can send email attachments from the Digidoc app but that is not a usable workaround in all scenarios. This means that iOS devices cannot be used for official correspondence in a state that is widely recognized as the global forerunner of e-governance and e-economy.