This page deals with the sending of attachments - look here for details of how to work with attachments you receive.
The Internet mail system can only convey text, it will not convey binary files, such as images, sound files, word processor files etc unless they are 'attached'. For more information on the technical details of this see here.
If you wish to send one or more such files with an email, press the 
button, and a dialog will open to allow you to search for and select the required file(s). You can select as many files as you like at once by the usual Windows method using the SHIFT or CONTROL keys. If you later want to attach one or more additional files to the same message, you can repeat the process.
Alternatively, file(s) may also be dragged and dropped from Windows Explorer, onto the MailCOPA editor window in which you are working.
You can also drag files from Windows Explorer onto the main MailCOPA window, and a Compose New Editor will be opened, with the file(s) already attached.
It is also possible to drag file(s) onto the MailCOPA button on the Task Bar, but you must wait for MailCOPA to open up before dragging the file(s) onto the main window.
However you add the files, you will see them listed in the Attachments Pane, to the right of the address and subject details:
Right-clicking on a file in the Attachments Pane produces a Context Menu with various options on it, including that of deleting the attachment (removing it from the message). For more details of this see here.
You can set MailCOPA to zip all attachments on sending by checking the box above the attachments pane - all of the files will be placed into a single zip file.
Data and the Internet mail system.
The internet mail system was originally designed only to convey 7-bit data - ie ASCII codes up to 127. Each character is represented by a number, eg a space is number 32, a capital A is number 65. Some machines will convey 8-bit data (ANSI codes up to 255 - used for extended characters such as é, ä, ß, © ) but this is not guaranteed, and 8-bit characters may arrive in a garbled form.
For sending and receiving such characters, MailCOPA supports a standard method called Quoted-Printable, which converts 8-bit characters to a string of 7-bit characters, which the receiving software (including MailCOPA) will convert faithfully back to the original 8-bit characters.
Binary data files can be 'attached' to a message by converting them into strings of 7-bit characters by a standard method. This is done transparently by MailCOPA, you don't need to sit poring over bits and bytes with an abacus!. Again, the receiving software (including MailCOPA) will translate the 7-bit characters back into the original binary form.