SUMMARY
View a folder in dolphin that contains *.iso files. The file type of some are show as "plain text document". Some are shown correctly as "raw CD image". By using the "file" command in a terminal I see the difference. "file" says that some iso files are:
ISO 9660 CD-ROM filesystem data 'sysrcd-5.3.1' (bootable)

while others are like this:
DOS/MBR boot sector; partition 2 : ID=0xef, start-CHS (0x3ff,254,63), end-CHS (0x3ff,254,63), startsector 4832, 18004 sectors

STEPS TO REPRODUCE
1. dolphin
2. folder with iso files in it
3. view details and "Type" column

OBSERVED RESULT
some iso files are identified as text and show a text file icon

EXPECTED RESULT
iso files should all be identified as iso and show the iso icon


SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 
KDE Frameworks 5.45.0
Qt 5.9.4 (built against 5.9.4)
The xcb windowing system

ADDITIONAL INFORMATION
openSUSE Leap 15.0