ColdFusion adding extensions to names of uploaded files?

So I was just working on some ColdFusion code to validate upload filename extensions, and I've noticed that when I delete the extension from a filename and then upload the file, the extension gets added back somehow I'm guessing based on the MIME type, however I'm not sure if ColdFusion is doing this, or the browser (MS Edge on Mac)? The extension appears inside the CLIENTFILE, CLIENTFILEEXT, CONTENTSUBTYPE, as well as the ATTEMPTEDSERVERFILE, SERVERFILE, and SERVERFILEEXT properties of the <cffile> result, even though the filename of the uploaded file didn't have an extension.

Anyone know what's going on?

This is the form input <input type="file" name="attachment1" style="width:100%"> and this is how it's being saved <cffile action="upload" filefield="attachment1" destination="#request.email_upload_folder#" nameconflict="overwrite" result="my_result1">. The extension also shows up when I use this function to get the client file name (I admit to copy-pasting this code without understanding how it works):

<cffunction name="getClientFileName" access="public" returntype="string" output="false" hint="">
  <cfargument name="fieldName" required="true" type="string" hint="Name of the Form field" />

  <cfset var tmpPartsArray = Form.getPartsArray() />

  <cfif IsDefined("tmpPartsArray")>
    <cfloop array="#tmpPartsArray#" index="local.tmpPart">
      <cfif local.tmpPart.isFile() AND local.tmpPart.getName() EQ arguments.fieldName>
        <cfreturn local.tmpPart.getFileName() />
      </cfif>
    </cfloop>
  </cfif>

  <cfreturn "" />
</cffunction>