I would like to cat the contents of a `file.txt` and use this as part of the `--data` field in my curl query.

I tried:

```
curl ... --data '{ "file_content":[" $(cat file.txt) ", ...]}"' ...

```

but that passes the string `$(cat file.txt)` to the server rather than interpreting it before sending.

I also tried:

```
curl ... --data '{ "file_content":[" `cat file.txt` ", ...]}"' ...

```

Again, the passes the string as it is instead of interpreting it.