java.net.URLConnection.guessContentTypeFromStream and text/plain

All,

I am trying to identify plain text files with Mac line endings and, inside an InputStream, silently convert them to Windows or Linux line endings (the important part is the LF character, really). Specifically, I'm working with several APIs that take InputStreams and are hard-locked to looking for \n as newlines.

Sometimes, I get binary files. Obviously, a file that isn't text-like shouldn't have this substitution done, because the value that happens to correspond to \r obviously can't silently be followed by a \n without mangling things badly.

I am attempting to use java.net.URLConnection.guessContentTypeFromStream and only performing endline conversions if the type is text/plain. Unfortunately, "text/plain" doesn't seem to be in its gamut of return values; all I get is null for my flat text files, and it's possibly not safe to assume all unidentifiable files can be modified.

What better library (preferably in a public Maven repository and open-source) can I use to do this? Alternatively, how can I make guessContentTypeFromStream work for me? I know I'm describing an inherently hazardous application and no solution can be perfect, but should I just treat "null" as likely to be "text/plain" and I simply need to write more code myself to look for evidence that it isn't?