How to upload a file to a WordPress site via the XML-RPC API. Pretty easy, no? The attachment page if you want a html wrapper to present the file can be found at the following URL: http://yoursite.com/?attachment_id=722 (I left the ID the successful response in as an example)

command.xml

<?xml version='1.0' encoding='utf-8'?>
<methodCall>
	<methodName>wp.uploadFile</methodName>
	<params>
		<param><value><string>1</string></value></param>
		<param><value><string>username</string></value></param>
		<param><value><string>password</string></value></param>
		<param>
			<value>
				<struct>
					<member>
						<name>name</name>
						<value><string>filename.jpg</string></value>
					</member>
					<member>
						<name>type</name>
						<value><string>mime/type</string></value>
					</member>
					<member>
						<name>bits</name>
						<value><base64><![CDATA[---base64-encoded-data---]]></base64></value>
					</member>
				</struct>
			</value>
		</param>
	</params>
</methodCall>

curl.sh

curl --data @command.xml http://yoursite.com/xmlrpc.php

invalid-response.xml

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
	<fault>
		<value>
			<struct>
				<member>
					<name>faultCode</name>
					<value><int>403</int></value>
				</member>
				<member>
					<name>faultString</name>
					<value><string>Incorrect username or password.</string></value>
				</member>
			</struct>
		</value>
	</fault>
</methodResponse>

successful-response.xml

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
	<params>
		<param>
			<value>
				<struct>
					<member>
						<name>id</name>
						<value><string>722</string></value>
					</member>
					<member>
						<name>file</name>
						<value><string>filename.jpg</string></value>
					</member>
					<member>
						<name>url</name>
						<value><string>http://yoursite.com/wp-content/uploads/2013/05/filename.jpg</string></value>
					</member>
					<member>
						<name>type</name>
						<value><string>mime/type</string></value>
					</member>
				</struct>
			</value>
		</param>
	</params>
</methodResponse>