On the iphone, go to files, browse, edit and toogle the dropbox on.
You can now access dropbox in the add attachment function in emails.
An excellent and knowledgable answer. I mean, it's basically a labyrith to get to the end of this settings-chain, and while instructions by matt228 are clear and sufficient, I would never have found this on my own. Meanwhile, note that this will not resolve access to Dropbox in the iOS Gmail app on the iphone (at least not for me). My impression is that Google has restricted the Gmail app to only allow access to files in the camera app and their own Drive app, trying to force users away from using Dropbox. A strategy that doesn't work with me. I switched to using a different client on iPhone instead.
Hint: access your gmail mailbox from web client. Ex: if you access gmail.com in your browser from iPad or iPhone, the attach button will then show the option to get files from Dropbox. That’s it! Hope This workaround helps!
This is one solution. But we can't still attach a Dropbox file while using the gmail app.
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Solved! : See solution