In my experience, Export Field Contents of a Text field will preserve the carriage returns present in the field.
How are you determining what you are getting? Are you opening it up in a text editor and not seeing the line breaks? Perhaps your text editor is having trouble reading the text file exported by FileMaker's Export Field Contents. The Export Field Contents function will export using UTF16. Not all text editors support that.
What version of FileMaker are you using?
What operating system are you running?
What tool are you using to read the file exported from FileMaker?
Opening the file in Windows Notepad, and no returns are evident. Using Filemaker Pro Advanced 13.0v3 on Windows 7 Professional.
The fault lies with Windows Notepad and its failure to accommodate any other type of line ending convention beyond DOS.
Windows Notepad will only display line breaks if they are encoded as CRLF (two characters: carriage return followed by line feed).
Export field conents only uses a single carriage return (CR) to denote the line endings since this is how it encodes the line endings internally in the text fields. (This is the old Mac convention.)
It is odd that Windows Notepad was updated over the years to understand UTF16 instead of just ASCII and UTF8, but it is still crippled with regard to line ending conventions.
Try opening the file in Windows Wordpad or any third-party text editor. You can then see the line endings appearing normally.
On windows I suggest Notepad++ (freeware) as a more powerful text editor to inspect the exported result. You can also see how the text is formatted... aciii or UTF-8 or 16 and options like little endian and whatnot, in addition to line endings (mac/unix/windows).
He need calculated text to export, so change expression as
Substitute ( originalExpression ; Char(13) ; Char(13) & Char(10) )
is another sugestion.
not to dismiss any of the other replies...
I wrote an article on extracting the characters (as-is from a field). <http://filemakerhacks.com/2012/09/23/export-field-contents-as-utf-8>. See if it sheds any light.
you may indeed need to make each CR into CRLF, or use a text editor to "convert". But it really all depends on WHERE this needs to be read. Some systems require LF, CR or CRLF as end-of-line. See if you can find out what your needs may be, then use the appropriate export.
NOTE that plug-ins, AppleScript and VB script can also preserve characters as-is when writing out the contents of field(s).