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diff -up cups-1.6.4/backend/ipp.c.ipp-multifile cups-1.6.4/backend/ipp.c
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--- cups-1.6.4/backend/ipp.c.ipp-multifile	2013-09-25 14:06:32.027732925 +0100
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+++ cups-1.6.4/backend/ipp.c	2013-09-25 14:10:22.134679512 +0100
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@@ -1685,7 +1685,10 @@ main(int  argc,				/* I - Number of comm
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 	ippAddBoolean(request, IPP_TAG_OPERATION, "last-document",
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         	      (i + 1) >= num_files);
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-	if (document_format)
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+	if (num_files > 1)
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+	  ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_MIMETYPE,
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+		       "document-format", NULL, "application/octet-stream");
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+	else if (document_format)
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 	  ippAddString(request, IPP_TAG_OPERATION, IPP_TAG_MIMETYPE,
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 		       "document-format", NULL, document_format);
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