To read or write delimited content from a file using the .dat extension, you can just change the name of the file when configuring the Write File or Read File operators. For example, while the example might write a file named myfile.txt, if you want to write to a file named myfile.dat, just change the name of the file when configuring the Write File operator.
If you want to write fixed width content to a file, you will need to approach this in a different way as Expressor does not have operators that are designed to read fixed width content. Please let me know if this is what you are trying to do.
We will work on creating a tutorial for this, in the meantime, you'll want to create a dataflow with the following operators:
read file --> write QVX
For the write QVX operator, you'll want to configure a file connection and a schema. The file connection can be the same you used as the read file (will write to same directory path). Or you can create a new one.
For the schema, you will need to click on the action button to the right of the schema drop down and select the create from upstream option. This will 'inherit' the schema from the upstream operator and allow you to quickly create a schema for your QVX output.
Then provide a name for your QVX file and table name and you'll be all set. Let us know if you need any additional help.