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While posting a Timeline Update
Press the Attach button
Press Add attachments button
Select Add from Google Drive
The Google Drive file picker will appear
Select the file you want to add and press Select (You can only select a single file at a time)
To add another file, press Add Attachments
Press Post
The file will be saved to your story. Click it to open it.
While posting a Quick Update from the Homepage or a Progress Board
Launch the Quick Update Popup
Press the Add from Google Drive button
This launches the Google Drive file picker
Select the file you want to upload (you can only select one)
Press Select
Now the file is selected, press Publish once you have finished creating your story
The story is posted and the file is attached. Click it to open it.
While completing a Survey
Go the Complete Survey page
Press the Add attachments button
Press the Add from Google Drive button
This launches the Google Drive file picker
Select the file you want to add
Press Select
The file is added
To add another file, clickthe Add attachments button again
Once you are done, press Publish Now
The file is uploaded to the survey response