Hey there, @srangaza ...
That's a great question, but unfortunately I don't have a great answer for you. The fact is...you cannot attach files from your course "Files" or your saved user "Files" areas within Canvas. I just tested this as well, and when you try and attach a file, it's asking to look for a location on your computer...not a location from within "Files". So, you're going to need to keep a copy of your files "offline" in the event that you want to attach a file using the Canvas Inbox.
Next, I'll just make a quick mention that Canvas does not have e-mail. Their Inbox is an internal messaging system, but it is not the same as e-mail. For example, you cannot use the Canvas Inbox to send a message to a friend or family member ... because those folks most likely are not enrolled in the same course(s) that you are. In the same way, your friends and family cannot e-mail you a message that you would be able to view within your Canvas Inbox...again, because they aren't enrolled in Canvas courses like you are. However, you can set up your Canvas user notification settings so that you e-mailed a copy of messages you receive via the Canvas Inbox. Those notification settings are all covered in these two Guides:
Finally, you might want to consider submitting your thoughts about attaching files from your "Files" area as an Idea here in the Community so that folks could later vote on and evaluate it. Maybe this would be something that others would like to also see as a feature of Canvas. You can read more about creating your own Idea by visiting the following page and looking at the links under the heading "Ideas and Themes":
Instructure Community Guide - Instructure Community (canvaslms.com)
You can get started with creating your own idea by going to this page and clicking on the blue "View and Submit Ideas" button:
Canvas Ideas and Themes - Instructure Community (canvaslms.com)
Hopefully this will be of some help to you...even if it wasn't the response you were maybe looking for.
Take care...be well.