I have submitted a XLF source file to a project. However, the file does not get segmented with result Not translatable:

Completed Segment Asset step of task 1 in time [00:00:00] on engine [xxxx:clone#1550718827:engine#2] with result Not Translatable

However, the XLIFF 1.2. File Type is present in my environment. What might be the reason for this?

Make sure that the Mime Type for xliff and xlf files is present in the Mime Type table. Go to the Management > Administration > Customization page and select Mime Types. If you are submitting files in both XLIFF and XLF format, make sure that both extensions are added to the Mime Type. If the xlf extension is not added, add it in the Extension field and click on Add. This is how the Mime Type should look like:

The Mime Type includes only the xliff extension, but not the xlf extension. Therefore, this extension is not recognized by WorldServer:

This article is relevant:

Are XLF/XLIFF files supported by a dedicated File Type in WorldServer?