To attach a file, go to the particular record of the object to which you want to attach the file and click the custom link button for attaching a file
Make sure you have already created the custom link Button for attaching and added it to the Layout of the Salesforce object, to which you want to attach file files
Search for the particular object to which you want to attach the file.
Click on the respective link for attaching in the custom links section, this will take you to the attached page.
Select the storage server profile.
Use the “Upload” button to select the file you want to upload. Enter the comment for the attachment (Optional) Click on “Attach”.
Click on the “Upload” button again if you wish to upload more files.
Click on the “Done” button it will redirect you to the record page of the Salesforce instance.