It’s because your web server does not know about MindManager files. The download process looks at the first lines of a file to identify its type. A MindManager file says it is a zip file. Change the file extension of a MindManager file to .zip and you will see it contains several files. The data and configuration of the map is in the XML file, there will be style files and image files, as well as others.
If you think you have received a MindManager file which has the zip extension, change it to .mmap or one of the others shown below. It should then open in MindManager.
The solution is to add at least the first line of the following code to your .htaccess file in the home folder of your web server (or advise the web site owner). If you are having similar problems with recent versions of MS Office files, add the rest. This declares the file types to your web server and any browser that tries to download the file. Now the vistors will be offered the option to save the file with the correct extension or open with MindManager, if it is installed on their client.
Add the following to .htaccess:
AddType application/vnd.mindjet.mindmanager .mmp .mmap .mmpt .mmat .mmmp .mmas
AddType audio/mpeg .mp3
AddType application/vnd.ms-word.document.macroEnabled.12 .docm
AddType application/vnd.openxmlformats-officedocument.wordprocessingml.document docx
AddType application/vnd.openxmlformats-officedocument.wordprocessingml.template dotx
AddType application/vnd.ms-powerpoint.template.macroEnabled.12 potm
AddType application/vnd.openxmlformats-officedocument.presentationml.template potx
AddType application/vnd.ms-powerpoint.addin.macroEnabled.12 ppam
AddType application/vnd.ms-powerpoint.slideshow.macroEnabled.12 ppsm
AddType application/vnd.openxmlformats-officedocument.presentationml.slideshow ppsx
AddType application/vnd.ms-powerpoint.presentation.macroEnabled.12 pptm
AddType application/vnd.openxmlformats-officedocument.presentationml.presentation pptx
AddType application/vnd.ms-excel.addin.macroEnabled.12 xlam
AddType application/vnd.ms-excel.sheet.binary.macroEnabled.12 xlsb
AddType application/vnd.ms-excel.sheet.macroEnabled.12 xlsm
AddType application/vnd.openxmlformats-officedocument.spreadsheetml.sheet xlsx
AddType application/vnd.ms-excel.template.macroEnabled.12 xltm
AddType application/vnd.openxmlformats-officedocument.spreadsheetml.template xltx