Add load_contents attribute to data inputs

If set to a non-zero value will make Galaxy deposit the first n bytes of
content within in the job.<param_name>.<content> dict.
This is similar to the `loadContents` flag in the CWL input binding
which will load the first 64KB.
See https://www.commonwl.org/v1.0/CommandLineTool.html#CommandLineBinding

lib/galaxy/tools/parameters/basic.py

@@ -95,6 +95,7 @@ class ToolParameter(Dictifiable):
         self.name = self.__class__.parse_name(input_source)
         self.type = input_source.get("type")
         self.hidden = input_source.get_bool("hidden", False)
+        self.load_contents = int(input_source.get("load_contents", 0))
         self.refresh_on_change = input_source.get_bool("refresh_on_change", False)
         self.optional = input_source.parse_optional()
         self.is_dynamic = False

lib/galaxy/tools/parameters/wrapped_json.py

@@ -66,7 +66,11 @@ def _json_wrap_input(input, value, handle_files="skip"):
             if value:
                 if isinstance(value, list):
                     value = value[0]
-                return _hda_to_object(value)
+                json_value = _hda_to_object(value)
+                if input.load_contents:
+                    with open(str(value), mode='rb') as fh:
+                        json_value['contents'] = fh.read(input.load_contents).decode('utf-8', errors='replace')
+                return json_value
             else:
                 return None
         else:

lib/galaxy/tools/parser/xml.py

@@ -113,7 +113,7 @@ class XmlToolSource(ToolSource):
         return ((command_el is not None) and command_el.text) or None
 
     def parse_expression(self):
-        """ Return string contianing command to run.
+        """ Return string containing command to run.
         """
         expression_el = self.root.find("expression")
         if expression_el is not None:

lib/galaxy/tools/xsd/galaxy.xsd

@@ -2098,7 +2098,7 @@ parameter being described. All the attributes for the ``param`` element are
 documented below for completeness, but here are the common ones for each
 type are as follows:
 
-$attribute_list:name,type,optional,label,help,argument,refresh_on_change:4
+$attribute_list:name,type,optional,label,help,argument,load_contents:refresh_on_change:4
 
 ### Parameter Types
 
@@ -2364,6 +2364,13 @@ tool form just below the associated field to provide information about the
 field.</xs:documentation>
           </xs:annotation>
         </xs:attribute>
+        <xs:attribute name="load_contents" type="xs:integer">
+          <xs:annotation>
+            <xs:documentation xml:lang="en">Number of bytes that should be
+loaded into the `contents` attribute of the jobs dictionary provided to Expression
+Tools. Applies only to type="data" inputs.</xs:documentation>
+          </xs:annotation>
+        </xs:attribute>
         <xs:attribute name="value" type="xs:string">
           <xs:annotation>
             <xs:documentation xml:lang="en">The default value for this