Thanks! That is the kind of thing I'm looking for. So apps rather than just clouds can also function as locations for attachments, that's good. Just have to wait for more apps to be updated.
The standard Mail app from Apple does currently support e.g. iCould Drive, Dropbox, Google Drive, Documents 5 and OneDrive to grab attachments.
Other apps like Spark and CloudMagic also offer support for e.g. Box but not with e.g. Documents 5 if you start with their app to write an e-mail and attach the file. When you would open Documents 5 it's ofcourse possible to use your favorite e-mail app. I know only a select few apps that really attach the file itself and do not supply a link in the e-mail to the file which are Spark, CloudMagic, Boxer and Apple's own Mail app.
Check out this topic which i started in September which offers quite a few apps that support attaching files to e-mail. I love Spark on my iPhone and i use CloudMagic on my iPad (since Spark is not yet available on iPad yet).
The standard mail app from Apple does have a very slow way (press in e-mail message > press twice on '>' press 'add attachment' > select storage > attach a single file) of attaching files (per file) since iOS9. With Spark i can even attach multiple files at once from various source. Is Spark perfect would you ask? No but it comes close. See also the other topic i mentioned for my remarks on all these apps.
Yeah it's a shame the iOS 9 email client only allows you to attach one document at a time but it allows you to add documents edited in different apps to one email, such as one from word and the other from excel.