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1. Navigate to and open the page in which you want to add a link. 
2. In the page you want to change, click Edit. 
3. Click the link icon. The Insert link dialog box appears. 
4. Select Files and select the attachment. 
5. Click Insert. A link appears, with the filename as the link text, as shown in the following example: Example_document_508_compliant.doc. 
6. To display the link as a thumbnail, click the link and select Show as thumbnail. The thumbnail replaces the link text, and it is still a link, as shown in the following example: 
   

   View file
   name Example_document_508_compliant.doc height 250

Also refer to the following Atlassian help page:

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