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.
PUBLISHERPOSTEnumerateUserSubscribedFiles
https://partner.steam-api.com/ISteamRemoteStorage/EnumerateUserSubscribedFiles/v1/
NameValueTypeRequiredDescription
uint64 Yes SteamID of user
uint32 Yes appID of product
uint32 NoEUCMListType
POSTGetCollectionDetails
https://api.steampowered.com/ISteamRemoteStorage/GetCollectionDetails/v1/
NameValueTypeRequiredDescription
uint32 Yes Number of collections being requested
uint64[] Yes collection ids to get the details for
POSTGetPublishedFileDetails
https://api.steampowered.com/ISteamRemoteStorage/GetPublishedFileDetails/v1/
NameValueTypeRequiredDescription
uint32 Yes Number of items being requested
uint64[] Yes published file id to look up
GETGetUGCFileDetails
https://api.steampowered.com/ISteamRemoteStorage/GetUGCFileDetails/v1/
NameValueTypeRequiredDescription
uint64 NoIf specified, only returns details if the file is owned by the SteamID specified
uint64 Yes ID of UGC file to get info for
uint32 Yes appID of product
PUBLISHERPOSTSetUGCUsedByGC
https://partner.steam-api.com/ISteamRemoteStorage/SetUGCUsedByGC/v1/
NameValueTypeRequiredDescription
uint64 Yes SteamID of user
uint64 Yes ID of UGC file whose bits are being fiddled with
uint32 Yes appID of product to change updating state for
bool Yes New state of flag
PUBLISHERPOSTSubscribePublishedFile
https://partner.steam-api.com/ISteamRemoteStorage/SubscribePublishedFile/v1/
NameValueTypeRequiredDescription
uint64 Yes SteamID of user
uint32 Yes appID of product
uint64 Yes published file id to subscribe to
PUBLISHERPOSTUnsubscribePublishedFile
https://partner.steam-api.com/ISteamRemoteStorage/UnsubscribePublishedFile/v1/
NameValueTypeRequiredDescription
uint64 Yes SteamID of user
uint32 Yes appID of product
uint64 Yes published file id to unsubscribe from