Files that have .bms at the end of there name are maps. You can only load .bms files in delta force land warrior. But you can't edit .bms files. .mis files are missions and can be edited in the editor by clicking file => open => select file. If you want to play a .mis file in delta force you need to open it in the editor then you click file and export binary mission now you type a file name and the editor will convert your .mis file to a .bms file. If you want to edit a .bms file you need to convert it using bms to mis convertor download here.
NOTE!! never convert .mis files to .bms or .bms file to .mis files by changing the filename in Windows explorer!!