Remember, access to the ClipBoard on browsers level is highly privileged security related action and may not work at all depending on system or browser you use! Only pure text and images are supported for copy/paste!
1. to paste clipboard text data to session use Ctrl + V, this works mostly on all browsers, on Mac browsers Ctrl button has no effect, there you must use it with Right_CMD + V (left_cmd button not accepted!)
2. to copy text data to your local environment use Ctrl + C or to cut Ctrl + X, on Mac Ctrl has no effect, use it with Right_CMD + C or Right_CMD + X
(Mac Safari has bug on older versions and does not wait for data from server on privileged action or adds data only on second Right_CMD + C pressing)
3. to copy data by mouse to your local browser environment is very tricky and requires browser support of ClipBoard API. On each mouse click you give the system grace time of 1 second, if in this time the data arrives from the server and ClipBoard API is supported then this data will be added to your system, but if the data was to late because of slow speed between server or data amount was just to big then this data will be added to server on next(second) click but only if ClipBoard API supported.
4. there is no technical way to paste clipboard data by mouse click from your local environment and send it to server therefore use Ctrl + V (or on Mac Right_CMD + V).
Chrome and FireFox considered it as big potential security risk and do not allow that even on privileged action like mouse click. However Internet Explorer partially supports such onclick pasting but with displaying of security message which when not accepted will break future clipboard input therefore this approach is not implemented for IE since considered as unstable.
PS: copying files between browser and RDP session via ClipBoard access by Ctrl + V or by mouse click is technically not possible!
Use drag-and-drop instead to send files to session and send files back to browser via \\tsclient\WebServer
ClipBoard image usage with HTML5 client.
By image copy/pasting you get same limitations like on ClipBoard text transfer but with even more restrictions.
1. To paste image data from your computer into RDP session just press Ctrl + V (on Mac Safari has bug and does not allow to access such pasted images therefore use Mac Chrome or Mac FireFox if you need to paste images on Mac)
(there is no way to paste images by mouse click at all)
2. To copy image to your local environment is tricky, you may however use Ctrl + C, but this will not add the image from inside RDP session to your local clipboard automatically.
Ctrl + C will just initialize sending of image clipboard data to browser, with same success you may initialize image clipboard data transfer with the mouse. When image arrives on your browser you will see shortly small preview of copied image area on top right side of your browser, as soon you saw that image arrived indicated by preview just make next click with right mouse button, this will initialize browsers native handler for right click. From that menu you must choose to copy the image to your local clipboard environment. Depending on browser you will see different menus and entries of the native handler for right click.
Actually image clipboard transfer was successfully tested on modern/newest versions of IE11, IE12(Edge), Chrome, FireFox, and on Mac browsers too (Chrome, FireFox). However on Mac Safari as mentioned you can not paste images due Safari'ssecurity bug!