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CER - CRT Files
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These files should be used if your server requires you to browse and upload your certificates into their system. Common servers that use these files are IIS, Exchange, and Plesk. 

The files in this folder are .CER files. If you need .CRT files, simply use a file explorer and manually change the file extension from .CER to .CRT and you'll be all set. 

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Plain Text Files
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These files should be used if your server or hosting provider gives you a form to copy/paste your certificates into. Common hosting providers and servers that require these kinds of files are cPanel, GoDaddy, HostGator, and WHM. 

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PKCS7 File
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This file should be used if your server requires that you upload a PKCS7 or .p7b file format for installation. Common server types that require this kind of file are Azure and TomCat.