This is impossible to do just by merely using the <input type="file">
with the ' 'multiple' keyword. This by itself will only allow attaching multiple files from same folder.
To complete your requirement, you will probably have to resort to using jQuery along with JavaScript and HTML, to provide an additional 'Upload' button after one or more files from the same folder are attached. This way you can attach any additional files from other folders and provide some sort of a 'Save' button, to eventually attach all files and save the result.
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