STASH Diff tool does not show changes for .F90 files

When viewing a 'Diff' of a source file, either through the branch comparison tool or in pull requests, files with a .F90 extension (not sure if it applies to .F) are not displayed as having changes in them. See the attached screenshot. The file is listed correctly in the 'Changed files' list but the Diff pane shows the icon with 'The contents for this file have not changed'. If you go to a source view of the file directly from its branch and and use the 'Diff to previous' button with the history that works fine ... so the changes expected are definitely in the repository.

The root of the problem seems to be, and this can be seen from the screenshot, is that although in the 'Changed files' list the filename is correct, in the header bar for the 'Diff' pane its extension has gone missing! If you click on the 'raw file' icon this confirms that it is trying to look for the file minus its extension. This only seems to affect .F90 files and not .f90 ... so the capital F seems to be important but can't think why. This makes reviewing changes to .F90 files very tricky.

This wasn't an issue in 2.12.0 the last version we had before upgrading to 3.1.0. I have tried this in IE and Firefox and it is present for both browsers.