Upload attachment to SharePoint list using HTML form and JavaScript

Sriniaghalaya 

You can use SharePoint REST API to upload attachment to SharePoint list items. You have to use below endpoint to do this:

/_api/lists/GetByTitle('listName')/items(<itemId>)/AttachmentFiles/add(FileName='<file name with extension')

check below links for detailed information:

1. Upload attachment to new item on list using REST API JQuery in SharePoint 2013 
2. Uploading Attachments to SharePoint Lists Using REST 
3. Add attachment to a list item using REST API 

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