Notes (Please be sure to check in advance) About the form tag Be sure to set the value of the name attribute of the file attachment part of the HTML on the form page side to "upfile[]" as shown below . Since multiple attachments on the HTML side are supported, even if there is only one name attribute value, it is necessary to add [ and ] ("and" is not required) after the value.
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Also, be sure to add enctype="multipart/form-data" to the form tag on the form page side as shown below. This is a required attribute when attaching a file.