function AttachmentHasKey(Index: Integer; const Key: WString): Boolean;
| Index | Integer. Zero-based index of the document-level attachment, below AttachmentCount. |
| Key | WString. Name of the file-specification dictionary entry, written as a PDF name. |
| Result | Boolean. True when the attachment carries that key. |
The file-specification dictionary of an embedded file (ISO 32000-1 7.11.3) holds the standard entries plus any private ones a producer adds. This probes for one key without reading it, which is how a caller distinguishes "absent" from "present but empty"
An index outside the attachment list, or a PDFium build without the capability, returns False rather than raising
Read and write the value with GetAttachmentStringValue and SetAttachmentStringValue