Here's the ExpressionTool version:

$ cat u.cwl 

#!/usr/bin/env cwl-runner
cwlVersion: v1.0
class: ExpressionTool

requirements: { InlineJavascriptRequirement: {} }

inputs:
  a_File:
    type: File
    inputBinding:
      loadContents: true

expression: |
  ${ return { "a_string": inputs.a_File.contents }; }

outputs:
  a_string: string

And it's output

$ echo "Hello, World!" > message
$ cwltool u.cwl --a_File message 
/home/michael/cwltool/env/bin/cwltool 1.0.20171107133715
Resolved 'u.cwl' to 'file:///home/michael/u.cwl'
{
    "a_string": "Hello, World!\n"
}
Final process status is success

And a third way, that converts any size of File that will fit into system memory into a CWL string. This is achieved using the JSON feature from http://www.commonwl.org/v1.0/CommandLineTool.html#Output_binding

$ echo 'Hello,
World!' > message

Our input file

$ cat v.cwl 
#!/usr/bin/env cwl-runner
cwlVersion: v1.0
class: CommandLineTool

inputs:
  a_File: File

baseCommand: python
arguments:
 - prefix: -c
   valueFrom: |
     import json
     a_string = ""
     with open("$(inputs.a_File.path)", "r") as a_File:
         a_string = a_File.read()
     with open("cwl.output.json", "w") as output:
         json.dump({"a_string": a_string}, output)

outputs:
  a_string: string

The CWL CommandLineTool itself

$ cwltool ../v.cwl --a_File message
/home/michael/common-workflow-language/env/bin/cwltool 1.0.20171107133715
Resolved 'v.cwl' to 'file:///home/michael/v.cwl'
[job v.cwl] /tmp/tmp4YLbgd$ python \
    -c \
    'import json
import os
a_string = ""
with open("/tmp/tmp3xHM90/stgdc883f3a-b4e7-4eab-aa42-3cddb08a822b/message", "r") as a_File:
    a_string = a_File.read()
with open("cwl.output.json", "w") as output:
    json.dump({"a_string": a_string}, output)'
[job v.cwl] completed success
{
    "a_string": "Hello,\nWorld!\n"
}
Final process status is success

And the CWL File to CWL string converter in action. As you can see, this reads the entire file into memory.