This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
UNDOCUMENTEDPOSTBeginAppUploadBatch

Indicate a batch of files that will be uploaded / deleted for an app.

https://api.steampowered.com/ICloudService/BeginAppUploadBatch/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 NoApp ID for this batch is for.
string NoClient machine name (may be user's alias).
string[] NoFilenames of files to upload in this batch
string[] NoFilenames of files to delete in this batch
uint64 NoClient ID number
uint64 NoCurrent local build of the app which made these changes
string Yes OAuth access token for the user
string list Yes List of all files to be uploaded (new files or updates for existing files) as part of this batch.
string list Yes List of all files to be deleted from Steam Cloud as part of this batch.
UNDOCUMENTEDPOSTBeginHTTPUpload

Begins the process of uploading a file to Steam external storage services. File should be uploaded via HTTP PUT to the returned URL, after which the upload must be finalized by a call to CommitHTTPUpload.

https://api.steampowered.com/ICloudService/BeginHTTPUpload/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 NoApp ID for which the file is being uploaded.
uint32 NoOriginal file size in bytes.
string NoName of the file to store in the cloud.
string NoHex string (40 digits) representing the SHA1 digest of the file.
bool NoTrue if the file should be marked public on the UFS, false otherwise.
string[] NoArray of string specifying which platforms to sync; value values: all, Windows, MacOS, linux, Switch, iPhoneOS, Android.
string[] NoNames for headers you'll want to set on your upload request. May be left blank.
string[] NoValues for headers you'll want to set on your upload request. The number of names must equal the number of values.
uint64 NoID of this batch returned by prior BeginAppUploadBatch call.
UNDOCUMENTEDPOSTBeginUGCUpload

Begins the process of uploading a file to Steam external storage services. File should be uploaded via HTTP PUT to the returned URL, after which the upload must be finalized by a call to CommitHTTPUpload.

https://api.steampowered.com/ICloudService/BeginUGCUpload/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 NoApp ID this UGC belongs to.
uint32 NoFile size in bytes.
string NoName of the file.
string NoHex string (40 digits) representing the SHA1 digest of the file.
string NoMIME content type of the file
UNDOCUMENTEDPOSTCommitHTTPUpload

Commits a file upload initiated by BeginHTTPUpload and transferred via HTTP PUT.

https://api.steampowered.com/ICloudService/CommitHTTPUpload/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
bool NoTrue if the HTTP PUT to the upload URL succeeded (URL provided in response to Cloud.BeginHTTPUpload), false if a failure occurred.
uint32 NoApp ID for which the file is being uploaded. Must match the app ID provided to Cloud.BeginHTTPUpload.
string NoHex string (40 digits) representing the SHA1 digest of the file. Must match the SHA1 digest provided to Cloud.BeginHTTPUpload.
string NoFilename as specified in the Cloud.BeginHTTPUpload request.
UNDOCUMENTEDPOSTCommitUGCUpload

Commits a file upload initiated by BeginUGCUpload and transferred via HTTP PUT.

https://api.steampowered.com/ICloudService/CommitUGCUpload/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
bool NoTrue if the HTTP PUT to the upload URL succeeded (URL provided in response to Cloud.BeginUGCUpload), false if a failure occurred.
uint32 NoApp ID for which the file is being uploaded. Must match the app ID provided to Cloud.BeginUGCUpload.
fixed64 NoUGC ID of the uploaded file.
UNDOCUMENTEDPOSTCompleteAppUploadBatch

Indicate that the batch is complete or being stopped for some other reason.

https://api.steampowered.com/ICloudService/CompleteAppUploadBatch/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 NoApp ID for this batch is for.
uint64 NoID of this batch.
uint32 Noresult of this batch.

Indicate that the batch is complete or being stopped for some other reason.

https://api.steampowered.com/ICloudService/CompleteAppUploadBatchBlocking/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 NoApp ID for this batch is for.
uint64 NoID of this batch.
uint32 Noresult of this batch.
UNDOCUMENTEDPOSTDelete

Deletes a file from the user's cloud.

https://api.steampowered.com/ICloudService/Delete/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
string No
uint32 NoApp ID the file belongs to.
uint64 NoID of this batch returned by prior BeginAppUploadBatch call.
UNDOCUMENTEDGETEnumerateUserFiles

Enumerates Cloud files for a user of a given app ID. Returns up to 500 files at a time.

https://api.steampowered.com/ICloudService/EnumerateUserFiles/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32 NoApp ID to enumerate the files of.
bool NoGet extended details back on the files found. Defaults to only returned the app Id and UGC Id of the files found.
uint32 NoMaximum number of results to return on this call. Defaults to a maximum of 500 files returned.
uint32 NoStarting index to begin enumeration at. Defaults to the beginning of the list.
string Yes OAuth access token for the user
UNDOCUMENTEDGetFileDetails

Returns details on a Cloud file.

https://api.steampowered.com/ICloudService/GetFileDetails/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 NoID of the Cloud file to get details for.
uint32 NoApp ID the file belongs to.