cfhttp post to AWS S3 works in CF9 but fails in CF10

I'm using the Instructure Canvas API for uploading files. After the initial API call, Canvas returns information that is used to make a POST request to Amazon Web Services's S3 service to upload a file.

Here is the request I'm using to post the file to AWS S3, populated with the data returned by the Canvas API:

<cfhttp url="#aws.upload_url#" result="result" method="post" multipart="yes">          <cfhttpparam type="formfield" name="key" value="#aws.upload_params.key#" />          <cfhttpparam type="formfield" name="acl" value="#aws.upload_params.acl#" />          <cfhttpparam type="formfield" name="Filename" value="#photoName#" />          <cfhttpparam type="formfield" name="AWSAccessKeyId" value="#aws.upload_params.AWSAccessKeyId#" />          <cfhttpparam type="formfield" name="Policy" value="#aws.upload_params.Policy#" />          <cfhttpparam type="formfield" name="Signature" value="#aws.upload_params.Signature#" />          <cfhttpparam type="formfield" name="Content-Type" value="#aws.upload_params['content-type']#" />          <cfhttpparam type="formfield" name="success_action_redirect" value="#aws.upload_params['success_action_redirect']#" />          <cfhttpparam type="file" name="file" file="#fullPath#" /></cfhttp>

This request works on a ColdFusion 9 server (the response returns "200 OK"), but fails on a ColdFusion 10 server (the response returns "500 Internal Server Error"). The Canvas API call works in both cases.

Any ideas or suggestions appreciated.