How To attach a file

Use the "File attachments" option at the bottom of the page, just above the Save and Preview buttons when you're typing your post. Click on Choose a file

it is also allowed to attach a backup file (starting with a dot, ending with a comma).
Remember, after selecting the file, to use the Save button.
After attaching the file, in the case of an image, you can insert it in the message.
Move your cursor in your reply (or post) where you want the picture then click the insert link (the underlined blue word insert)

That then also disables the view of the attachmant itself

Allowed file formats: jpg jpeg gif png txt doc xls pdf ppt pps odt ods odp xml mxl msc mscz mscx mscz, mscx, docx musicxml ts mid midi ly zip eps patch svg ove drm mss cap gp3 gp4 gp5 gpx gp qml tg 7z bww capx mpal kar sgu scw gtp gt3 gt4.

The maximum allowed size is 4 MB.

Compress the file using zip or 7z for files exceeding that size or not being among the allowed extensions.