For files such as spec sheets, photos, documents etc. you would like to re-use on your proposals, you can follow the steps described below:
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Go to Uploads tab and click "tap or drag here to upload" files.

Make sure you enter a tag to easily reference that file later on other places in the app then click start upload. You can upload multiple files at once or one by one using the same method.

Make sure your file is uploaded and tagged correctly. If not (or if you want to edit the tag) you can click the gear icon below the thumbnail and do the changes:

Go to edit a proposal / canned proposal solution (or create a new one) and click "Archive" button on attachments section:

Search for the files from the archive using the tag, when you find them click "Select" below their thumbnails:

Finally click "Save Files" button to add them to your proposal solution:

Your proposal solution now shows the file you uploaded:

Note: You can always directly upload files to your proposal solutions however these files are stored linked to those proposals and they will be deleted when you delete that proposal.