If you are looking to embed a file link in the email templates, first of all you need to upload the file into Salesforce Documents or any other file storage system which can provide you a public URL for the file.
Following is the article that explains how to upload a file to Salesforce Documents and open it as a public file to get the public file URL.
https://help.salesforce.com/articleView?id=docs_upload.htm
Once you have uploaded and got the public URL for the file now you can embed a file link in the MassMailer Email Editor.
Following screenshots explain the process.
You can go to the MassMailer Editor from the steps you typically follow when you are sending mass emails either through Campaign or the Email Wizard.
You can load the email template and select to update the template. You can add file link to any text in the email template by selecting the text and clicking on the Link icon on the Editor Editing Options.
In the popup window, you can enter the public file URL and click Ok to Save it.
Now the text that was highlighted in the Editor shown as an anchored text with a link to it. When the recipients receive the email, they will get the email with the file attachment link in it. They can click the link to open the File.
MassMailer has the capability to track the clicks of the links. You can see them as the regular click event in the email statistics. You can also find the clicked URL from these events.