Trying to invoke candidate attachment REST API through PL SQL procedure giving error

Summary:

Trying to invoke candidate attachment REST API through PL SQL procedure giving error " PLS-00306: wrong number or types of arguments in call to '||' "

v_buffer1 := '{
"DatatypeCode": "FILE",
"FileName": "Iron Man Resume (1).docx",
"DmFolderPath": null,
"Url": null,
"CategoryName": "MISC",
"Uri": null,
"UploadedText": null,
"UploadedFileContentType": "application/text",
"UploadedFileLength": 10164761,
"UploadedFileName": null,
"ContentRepositoryFileShared": "false",
"Title": "test.jpg",
"Description": "",
"ErrorStatusCode": null,
"ErrorStatusMessage": null,
"FileContents":"'||i.DOCUMENT_CONTENT||'"}';

Please assist with this error.

Thanks,

Saurabh Mharolkar