For certain types of files uploaded to Amazon S3, you may need to request that Amazon serve the file a certain way by using custom headers.
To set custom headers for S3 files:Choose Transmit > Preferences...
Click Cloud
To add default headers for all S3 uploads, click Default, otherwise click +/- to add your own file type extensions, such as "mp3", "zip", etc.
Click + to add a header for the selected extension, or click and hold + to choose from a list of commonly used headers.
Type in the value you wish to set for that header.
Use the checkbox next to the header to enable or disable that particular header.