net.http: fix multipart/form-data file upload handling

examples/vweb/file_upload/index.html

@@ -1,6 +1,15 @@
 <h2>File Upload</h2>
 
+File form:
 <form method="POST" enctype="multipart/form-data" action="/upload">
-	<input type="file" name="upfile" multiple>
-    <input type="submit" value="Press">
+	<input type="hidden" name="file_id" value="aafueB5Quie" />
+	<input type="file" name="upfile" multiple/>
+	<input type="submit" value="Upload"/>
+</form>
+
+Text form:
+<form method="POST" enctype="multipart/form-data" action="/submit">
+	<input type="text" name="content1" value="abc" />
+	<input type="text" name="content2" value="xyz" />
+	<input type="submit" value="Submit content">
 </form>

examples/vweb/file_upload/upload.html

@@ -5,6 +5,7 @@ File amount: @fdata.len
 @for i, data in fdata
 
 <h2>Filename: @data.filename</h2>
+<h2>Length: @data.data.len</h2>
 <h2>Type: @data.content_type</h2>
 
 <p>@{files[i]}</p>