Fix chaining and add example

Author: Laurynas Butkus

convertapi/file_param.py

@@ -9,6 +9,9 @@ def build(resource):
     if isinstance(resource, Result):
         return resource.file.url
 
+    if isinstance(resource, ResultFile):
+        return resource.url
+
     if isinstance(resource, UploadIO):
         return resource.upload()
 

examples/conversions_chaining.py

@@ -0,0 +1,22 @@
+import convertapi
+import os
+import tempfile
+
+convertapi.api_secret = os.environ['CONVERT_API_SECRET'] # your api secret
+
+# Short example of conversions chaining, the PDF pages extracted and saved as separated JPGs and then ZIP'ed
+# https://www.convertapi.com/doc/chaining
+
+print('Converting PDF to JPG and compressing result files with ZIP')
+
+jpg_result = convertapi.convert('jpg', { 'File': 'files/test.pdf' })
+
+print("Conversions done. Cost: %s. Total files created: %s" % (jpg_result.conversion_cost, len(jpg_result.files)))
+
+zip_result = convertapi.convert('zip', { 'Files': jpg_result.files })
+
+print("Conversions done. Cost: %s. Total files created: %s" % (zip_result.conversion_cost, len(zip_result.files)))
+
+saved_files = zip_result.save_files(tempfile.gettempdir())
+
+print("File saved to %s" % saved_files)