Are XML files handled differently when creating Signed URLs?

We're using JWPlayer6 to stream our videos, and they include Closed Captions that are contained in an XML file. I use standard PHP function that creates the Cloudfront Signed URLs that works perfectly with MP4 and PNG files, but does not work with XML files:

$SignedUrl_mp4 = getAwsSignedUrl('myvideo.mp4')    // This Signed URL works 
$SignedUrl_png = getAwsSignedUrl('myimage.png')    // This Signed URL works 
$SignedUrl_xml = getAwsSignedUrl('mycapts.xml')    // This Signed FAILS

All files exist, and the captions work on the player if I open them from a non-Amazon server. From a browser, the XML signed URL returns "access denied", thus my conclusion that it's a Policy/Signing issue.

I understand that the routine to sign urls is different for RTMP streaming than it is for HLS, but I can find no documenation that there's any special handling of XML files.

(I've not included the code for getAwsSignedUrl() because it's large and cumbersome, and because by all accounts, it should process an XML file the same as a PNG file, thus that code doesn't affect the answer to this question)