Probably because seagulls14b.sqs, is infact seagulls14b.sqs.txt.
Windows by default hides the file extension when you view files. If you look at the mission.sqm, do you see the full name of the file mission.sqm, or do you just see mission?
If this is the case, at the top of the folder, click the Tools drop down menu button, then click Folder Options. This will open a new propeties window. Click on the View tab at the top of this new window, and scroll down till you find the section for hide file extensions for known file types, make sure that that is unchecked.
Then go back to your folder and your seagulls14b.sqs file, and rename the file, deleting any .txt extension from the end of it.