To attach a file to your post you either need to upload a file from your computer or you need a link.

Since we usually upload images in our posts, I will explain how an image is uploaded.
To get a screen shot follow the steps given HERE.

Now it's time to upload the image to your post.
Follow the steps given below.
1. Go advanced.
2. Scroll a little to the "Additional Options" and click on the "Manage Attachments" button.

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3. A new window will open called an "Upload Manger." To open this window you can even click on this paperclip () as seen in the image below.

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4. Next click the "Add Links" button in the upper left corner (looks like this)=> Click image for larger version

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This open a new box in the right corner. At the very top (near the black "X" in the right corner) of that new box you will see it is defaulted to move images from your computer, but you can choose the "website" option link for images from online (then back to "computer" if you change your mind).

From Website: Enter the full Url in the box provieded(http://myimage.com), then press the "Upload" button.

From Computer: Press the "Select Files" button. You will get a pop up window of what is on your computer. Browse for the image(s) you want, & then the "open" button.
-If you grab the wrong images you can press the "clear list option" if you want to remove a bunch of images.
-If you have just one image you want to remove, just select the "X" by that image (it turns red when you hover over it). This will delete the image for you.

Already Uploaded
You will see a list of images in the middle box these are here so you can find locations of images you already used/uploaded. You can choose to make the list smaller by selecting to get rid of ones not used in a hour. You can also reuse them by just grabbing the picture you want, & throwing it into the box below (be sure it's checked to be used once moved).
Once you have the image(s) you want, select the "Upload Files."

(Make sure the url is a valid link and make sure all images from your computer or online, have an extension of .gif, .jpeg, .jpg.)

Now that you have your image(s) you should see them in the bottom box under the "Attachments" menu bar. make sure the upper box of the image is checked so the image is selected.
Here is an image of what you should have so far:

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You can also use the "select all" option below to make sure you have them all (they will highlight for you). Once you are satisfied with your choices, select...
Insert Inline Button: to put the image anywhere inside your post
Done Button: If you just want it to be an attachment at the bottom of your post.
If the "Manage Attachments" window is still open, just close it.

Now under that area below your post, under "Additional Options" will display this:

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* This means that you images are uploaded and attached to your post. Submit the reply. Remember that maximum of 5 files are allowed in a single post. You get errors if you exceed this!
If you placed your image(s) inline. you have the option of moving it around. Just high light what you have in line, & click the align buttons from your post menu.

Use THIS this thread for testing images, if you want to practice.