Here’s how to set the default value for the FormatType list in the Capture Viewport dialog box.

from siutils import si		# Application
from siutils import log		# LogMessage
from siutils import disp	# win32com.client.Dispatch
from siutils import C		# win32com.client.constants

# Set the capture options
oViewportCapture = si.Dictionary.GetObject("ViewportCapture")
paramFormatType = si.Dictionary.GetObject( "ViewportCapture.FormatType" )

# Set default to JPEG
paramFormatType.Value = 5

# Display Viewport Capture dialog
si.CaptureViewport( 2, True )


# FormatType values
# -----------------
# 0 Alias
# 1 AVI
# 2 BMP
# 3 Cineon (DPX)
# 4 Cineon (FIDO)
# 5 JPEG
# 6 Memory mapped
# 7 mental ray color
# 8 OpenEXR
# 9 PGM
# 10 Photoshop PSD
# 11 Pict
# 12 PNG
# 13 PPM
# 14 Quicktime
# 15 SGI
# 16 Softimage .pic
# 17 Son Playstation2 TIM2/CLUT2
# 18 Targa
# 19 Tiff
# 20 Valve
# 21 Wavefront
# 22 YUV

If you are scripting a non-interactive viewport capture, you don’t use FormatType. Instead, the output file format is determined by the file extension of the output file name.
Here’s a Python version of the example on the CaptureViewport reference page.

from siutils import si		# Application
from siutils import log		# LogMessage
from siutils import disp	# win32com.client.Dispatch
from siutils import C		# win32com.client.constants
import win32com.client.dynamic

# Get the current frame
fc = si.ActiveProject.Properties("Play Control").Parameters("Current").Value

# Set the capture options
oViewportCapture = si.Dictionary.GetObject("ViewportCapture")

# Capture a 1-frame sequence 
start = oViewportCapture.NestedObjects("Start Frame")
win32com.client.dynamic.Dispatch(start).Value = fc

end = oViewportCapture.NestedObjects("End Frame")       
win32com.client.dynamic.Dispatch(end).Value = fc

# Specify the output file name
# For scripted captures, the output file format is determined by the extension
name = oViewportCapture.NestedObjects("File Name")
win32com.client.dynamic.Dispatch(name).Value = "C:\\test.jpg"

# No dialog
si.CaptureViewport( 2, False );