[Zope] Uploading image files via forms ...

[Darran Edmundson]

Imagine a ZClass that can optionally contain an
image.  You'll probably have something like the following
in your addForm method:

<INPUT TYPE="file" NAME="newimage" VALUE="">

In your ZClass add constructor, after the new instance is
created, we want to (conditionally) create the image only 
if a file was chosen. How?  A piece of code like the 
following doesn't work:

 <dtml-if newimage>
     <dtml-call "manage_addImage('id', newimage.read(), '')">
 </dtml-if>

When no file is chosen, Zope still creates an image (size 0)
because REQUEST.newimage is defined (albeit empty).  I
immediately tried

 <dtml-if "_.len(_.str(newimage)) !=0">
     <dtml-call "manage_addImage('id', newimage.read(), '')">
 </dtml-if>

but this still creates a size 0 image.  Why?  

Well, there's yet another possible way out of this mess 
as documented in the How-to
"http://www.zope.org/Members/Zen/howto/FormVariableTypes"

  <INPUT TYPE="file" NAME="newimage:string:ignore_empty" VALUE="">

with

 <dtml-if "newimage !=''">
     <dtml-call "manage_addImage('id', newimage, '')">
 </dtml-if>

(Note, the string typecast is necessary or else ignore_empty
seems to have no effect.)  This works in that not choosing
a file stops the image from getting created and an image
does get created if one was chosen.  The problem now is
that the string typecast has lost the content_type of the 
uploaded image.  If it was "image/gif", it now shows up
as a raw octet_stream.

If anyone can provide a way out of this morass, I'd greatly
appreciate it.

Cheers,
Darran "veteran newbie" Edmundson