Files do support multi line, so this should be reflected by the manual.
Novice user but it looks like your using a file source with no-multi-line have you tried sending using any of the suported drivers ?
no-multi-line: The no-multi-line flag disables line-breaking in the messages; the entire message is converted to a single line. Note that this happens only if the underlying transport method actually supports multi-line messages. Currently the
,syslog(), network(), unix
-dgram() drivers support multi-line messages
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