Certain file name causes 'Unsupported Content Type' error

I'm programming for Android with Eclipse.

I created 'res/anim' folder for Tween Animation,

and wrote 'page_in_left.xml', 'page_in_right.xml', 'page_out_left.xml' and 'page_out_right.xml' files in the folder.

In fact, those four files are all the same because I copied and pasted from one file.

But when I double-clicked on 'page_in_left.xml' and 'page_out_left.xml',

the 'Unsupported Content Type' dialog box pops up.

(On the other hand, rest of the two files have no problem.)

I deleted them and created them and then tried again several times but only to fail.

So, I tried to rename them.

I removed the first letters from the both files('age_out_left.xml'), and it worked.

I renamed them back to their original name, then the error came back again.

I tried to put them in other folders such as 'res/raw', and 'res/drawable'.

In those folders, I could open the files without any error messages.

I don't understand why this happens.

Anybody knows?

PS. I also Cleaned the project.