DFU Update says zip file not supported

[rider]

I'm trying to apply a firmware update using a zip file that has worked with older version of nRF Toolbox. Using the current version, (4.1.1) on iOS, I select DFU, then select my zip file under User Files. When I press Done, I get a popup saying "DFU -- Selected file is not supported".

Any ideas what to do? It's not my file, and I didn't have any development role, so I'm just trying to figure out how to apply the update.

Any help would be really appreciated.

The zip file: IRT_V2_REV_A1_C_1_0_39.zip

[rider]

Sorry I did not attached the zip file in the original post. That would have been most useful.I have added it to the end of the original post.

I don't know much (not the previous nRF version, SDK or SoftDevice version), as I am only told it used to work.

This was developed by a technology consultant to a very small non-technical manufacturer. They have since parted ways, and the company has no further development or support capabilities. However, they are able to apply the firmware upgrade using their Android phones. I only have iOS devices, and am unable to reproduce their success.

Once again, thanks for your help in solving this problem!