ftpd on-the-fly conversions incompatible with curl

I have been using the NetBSD ftpd server for some time specifically because of 
the on-the-fly conversion capability. Usually that is for gziping files 
before transmission, but I have a more critical need - I use it to process data 
for anonymous users. The NetBSD ftpd configuration file allows the 
available conversions to depend on additions to the filename. For example:

    conversion all .gz f . /usr/bin/gzip -c -- %s

will cause a file "foo" to be sent gzipped if file "foo.gz" is requested. 
This won't work with GUI ftp clients (foo.gz won't be in the directory 
listing), so I would advise my users to use curl instead.

However, it seems that curl inquires about the size of the file before "get"ing 
it, and ftpd replies "550 foo.gz not a plain file" when a file is subject 
to conversion, which is apparently treated as a fatal error by curl, and curl 
does not attempt to "get" the file. Here is a portion of the --verbose output 
from curl:

   > TYPE I
   < 200 Type set to I.
   > SIZE foo.gz
   < 550 foo.gz: not a plain file.
   * The file does not exist

According to this post (https://curl.se/mail/lib-2004-10/0035.html), if 
the SIZE command did not exist, curl would still work. It appears that 
existing and providing a specific error message is the getting in the way. 
I am not sure why curl bothers to issue the SIZE command even when the 
-ignore_content_length option is specified. It seems unnecessary.

Is there any way around this? There is information about curl option 
ignore_content_length at https://curl.se/mail/lib-2015-09/0121.html 
showing that it was added for the case of files that grew while being 
downloaded, a slightly different situation. In that case the filename 
requested does exist.

Daniel Feenberg
http://taxsim.nber.org/
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