Here's the request body:
#set($this = "
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="file"; filename="marketo.html"
Content-Type: text/html
<html>
<body>
<h1>Test Page - marketo.html</h1>
</body>
</html>
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="name"
marketo.html
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="folder"
{"id":436,"type":"Folder"}
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="description"
This is a test file
------WebKitFormBoundary2VyWOacQSupl4gUL—
")
Sent to the endpoint /rest/asset/v1/files.json
You'd highlight that code as Plain (as MIME isn't a supported syntax) like so:

Which turns it into:
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="file"; filename="marketo.html"
Content-Type: text/html
<html>
<body>
<h1>Test Page - marketo.html</h1>
</body>
</html>
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="name"
marketo.html
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="folder"
{"id":436,"type":"Folder"}
------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="description"
This is a test file
------WebKitFormBoundary2VyWOacQSupl4gUL—
The #set() syntax is literal Velocity/Java code. It's not part of using the hightlighter.
Now we can look at the payload properly and see there are several things wrong with it.
First, this boundary is incorrect for your payload (this is exactly why a monospace font is used for code):
multipart/form-data; boundary=------WebKitFormBoundary2VyWOacQSupl4gUL
You have 6 hyphens:
------WebKit...
And your actual MIME parts also have 6 hyphens, which is incorrect:
------WebKit...
In the body, you want 8 hyphens, the constant prefix of -- followed by the boundary.
Second, after the final boundary, you need to have an additional suffix -- (you have an em-dash there instead).
Third, you need to have a line break (CRLF) after the header section of each MIME part.
So the correct format is:
--------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="file"; filename="marketo.html"
Content-Type: text/html
<html>
<body>
<h1>Test Page - marketo.html</h1>
</body>
</html>
--------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="name"
marketo.html
--------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="folder"
{ "id":1187, "type":"Folder" }
--------WebKitFormBoundary2VyWOacQSupl4gUL
Content-Disposition: form-data; name="description"
This is a test file
--------WebKitFormBoundary2VyWOacQSupl4gUL--