The industry standard for Gerber files is an extension of .gbr. It is very unusual that your PCB Manufacturer is asking you for different extensions to the gerber files.

Normally all that is required is that you generate your gerber files with a meaningful name, Like Top.Gbr. Bottom.gbr etc.

If the board house is unable to work with .gbr as the standard file extension I suggest that you should consider a different board house to make your boards.

All the best.

I am sorry but this is not very useful. The .GBR extension may have been the standard used in the days of photo plotting when humans would print the gerbers onto films and then manually load them on the appropriate manufacturing machines according to the information printed on the films, but in the 21c most PCB manufacturers are completely automated with uploads checked by computer for compliance. These systems need to have ways of identifying the files and although meaningful names could be used these do not comply with company standards of traceable files if every file in each project has the same name and without version control, and still these names would have to be understandable by all manufacturers.

Alitium reconised this decades ago and came up with different extensions for different film types and stacking positions and this is used by many manufacturers as a preference. The manufacturer we use sates 

Suggested Naming Patterns

We do our best to anticipate the default naming schemes from many PCB design packages. However, if you have problems, this is a naming scheme that is known to work.

And adds

ORCAD/Cadence Allegro

ORCAD usually produces all gerbers with a .PHO or .ART extension. Our site does not parse this effectively, so the files must be manually renamed to the pattern suggested above.

If allegro gerbers were standard file extension then why the default is .ART?

As said above  17.2 worked without issues and I had not had this problem with 16.6.

I  was asking if there is something I missed as a number of things have changed between 17.2 and 17.4 and this is my first 17.4 since upgrading my CAD

I have found it. if you want different extensions for output files as per altium

1. Use the extensions as the film names such as GLT, GLB etc

2. In prefix add . (dot) to the end of your filename prefix such as "ABC."

3 In user preferences -> file management  -> versioning change ext_artwork to . (dot)

Artwork files then become ABC.GLT, ABC.GBL etc

I had to rebuild all my usr preference when I did a clean install, so may have missed out the ext_artwork, it had been such a long time since I had to set these up

Jessica

As you stated in the 21c ..  Gerbers are so 1980's, in the 21c we use IPC2581 files.