For a long time hledger has auto-detected the file format when it's
not known, eg when reading from a file with unusual extension (like
.dat or .txt), or from standard input (-f-), or when using the include
directive (which currently ignores file extensions).
Auto-detecting has been done by trying all readers until one succeeds.
This could guess wrong in some cases, but it was so rare that it has
been working fine.
Recently, more conveniences have been added to timedot format,
increasing its overlap with journal format, which makes this kind of
auto-detection unreliable.
Auto-detection and auto-detection failures are (probably) still pretty
rare in practice. But when it does happen it's confusing, giving
misleading errors or false successes (eg printing timedot entries
instead of a journal error).
For predictability and to minimise confusion, hledger no longer tries
to guess; when there's no file extension or reader prefix, it assumes
journal format. To specify one of the other formats, you must use a
standard file extension (.timeclock, .timedot, .csv, .ssv, .tsv), or a
reader prefix (-f csv:foo.txt, -f timedot:-).
For now, the include directive still tries to autodetect
(journal/timeclock/timedot), and this can't be overridden; it will be
fixed later.
Experimental; testing and feedback welcome.
b1f3880c3d