Use a Salesforce Attachment or Salesforce Files as a Conga Template
In addition to pre-selecting a Conga Template, Conga Composer can also gather attachments from the Notes and Attachments object related to the master object from which you are running your Conga Composer solution. This is helpful if there are additional documents that should be included with your merged document. To learn more about this process please review this Conga Blog post under number 2, "Using a Salesforce Attachment as a Conga Template."
The Composer Lightning component does not support it.
For example, you may want Conga Composer to generate a proposal document from an Opportunity record but also need to include the Terms and Conditions document that is stored as an attachment (under Notes and Attachments) on the Opportunity.
Below is an example of what your button URL might look like when the &QVar0Id and &TemplateID parameters are added.
In this example, {!Account.Id} is being passed into the pv0 filter of the query in order to retrieve the attachment(s) from the Account record related to the Opportunity. Furthermore, we're using the &QVar0Format parameter to format the output of the results retrieved in the Qvar0 query; this is necessary for situations where more than one attachment may be retrieved.
https://composer.congamerge.com?sessionId={!API.Session_ID} &serverUrl={!API.Partner_Server_URL_290} &id={!Opportunity.Id} &QVar0ID=a0Ai0000009jpZH &TemplateID=a0Bi000000MrkC8,{QVar0} &QVar0Format=10010 &DefaultPDF=1