Seems that files are encrypted with some kind of malware from crypto family. Generally it decrypt all doc, jpeg xls etc. files at infected computer.
And some kind of these also at accessible network shares.
Because not all files are encrypted and there was no warning note from malware ( the picture with warning over the desktop) or was there (or at another PC)?
And also because all/both folders with some encrypted files are accessible via network is possible that the infected PCs is some another in network.
So one option is – that at different PC the malware was detected “in the middle of the work” so did not finish its job and encrypt only some files (at some network shares). Or that malware still running somewhere and continue in encrypt.
I strictly recommend you to update virus definitions and run scan of all local disk at every computer in network. And check all other computers if there is any “exx” file at another ones – especially if you at some computer find ,exx file at C drive .etc – at some non-shared folders - you can be sure that it is origin of infection.