The extension on indexed data files may become an issue especially when coming from other legacy COBOL brands.
Here we will analyze the different alternatives isCOBOL provides to handle the extension and how it affects several areas of the application during migration, maintenance and deployment time.

First: Migrating indexed data files that already contain an extension (.dat for example).

Whether the indexed data file comes in a single .dat file or on a dual form of .dat and .index (the .index extension may vary from COBOL to brand to brand) the main case is when the SELECT statement explicitly and static or dynamically defines the physical name with the .dat extension.
Something like:

    select myfile assign to random "myfile.dat" ...

    select myfile assign to ws-filename ...
    procedure division. 
       Move "myfile.dat" to ws-filename 
       Open input myfile 

If ISMIGRATE is used in background (called from a cobol program), the following runtime property should be set:

iscobol.ismigrate_strip_extension=1

iscobol.file.index.strip_extension=1 

Second: Create an indexed data file from scratch using Jisam or C-tree directly.

In this case it's better to avoid the .dat extension in the physical filename on the select, so Jisam or C-Tree will by default generate the .dat extension for the data file part and the .idx on the index file.
For example this Select:

    select myfile assign to  "myfile" ...

iscobol.file.index.data_suffix=
iscobol.file.index.index_suffix=.fil

1. File Types Comparison : isCOBOL - Micro Focus COBOL - ACUCOBOL-GT - RM/COBOL
2. How do I interpret file status 9? and other EXFS extended file status codes (e.g. 9i,105)?
3. What format does isCOBOL use for ORGANIZATION RELATIVE files?
4. What is 9l,10 error and why is it happening?
5. Does isCOBOL support Data General file status codes?
6. Can I install the UDBC 32 bit on my 64 bit computer in order to have the Veryant * driver for ODBC32 bits Administrator?
7. Can I call a stored procedure from an isCobol program?
8. Are Veryant's isCOBOL JISAM data format the same as Faircom's c-treeRTG?
9. Why do I get 'ct_init ERROR 19:133:0' ?
10. Did you know you can use an alias to map a different disk file name to an existing "assign to" clause of the select statement without source code changes?
11. Did you know you can encrypt an indexed file?
12. How to hide the user and password used to connect to an RDBMS
13. Can we use Micro Focus format data and indexed files directly or do they need some form of conversion?
14. How to access files on a different server
15. How to create, write, and read to files without a program
16. Managing the order of the alternate keys in an indexed file