Database.query('select (Select Name, BodyLength, ContentType from Attachments), Id,Name from Expense__c where Custom_Object__c =: CustomObjectID')

gives response like <br>
Attachment:{Name=file2.jpg, BodyLength=658456, ContentType=image/jpeg}, <br>
Attachment:{Name=file1.txt, BodyLength=618, ContentType=text/plain}, <br>
Attachment:{Name=file.cls, BodyLength=117141}, <br>

Most likely the ContentType is null in the database. Salesforce infers the ContentType for "well known" file extensions such as .jpg and .txt when a file is attached but it looks like it does not have a built in file extension to ContentType mapping for .cls.

Correction: this https://stackoverflow.com/questions/1201945/how-is-mime-type-of-an-uploaded-file-determined-by-browser says it is the browser that works out the MIME type not the server.

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