If you send too many file attachments in Gmail, previously you would have found it cumbersome; but now it is very easy.
How to attach multiple files at once?
It is simple -- go to the folder where you want to attach multiple files and hold down the "Ctrl" Key in PC (Cmd on OS X in Mac computers) and click each file. You can either choose them in sequence or at random order within the folder as shown below:
If the files are in sequence, you can either use "Shift" or "Ctrl" key but if they are in random order then you can only use the "Ctrl" key.
As you attach the files, you will see a download bar status (as shown below), and while it downloads, you can then start compiling the message.
By the time you finish composing the message all the files would have finished downloading (depending on the size of the files).
I usually do the file attachments first before compiling the message; it saves time, because otherwise you have to wait for all the files to finish downloading.