You are getting a single '{' because im_file treats each line in the input as a separate event - i.e. that's what InputType LineBased is.
You need to use xm_multiline to read and concatenate the indented json into a single event record, this can be done similarly to the xml parsing example with something like this:
<Extension multiline> Module xm_multiline HeaderLine /^{/ EndLine /^}/ </Extension>
Make sure to change InputType so that this is used.